Thinking in Java, 3rd ed. Revision 2.0
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A fundamental design guideline is “make simple things easy, and difficult things possible.” [67]
The original design goal of the graphical user interface (GUI) library in Java 1.0 was to allow the programmer to build a GUI that looks good on all platforms. That goal was not achieved. Instead, the Java 1.0 Abstract Window Toolkit (AWT) produces a GUI that looks equally mediocre on all systems. In addition, it’s restrictive: you can use only four fonts and you cannot access any of the more sophisticated GUI elements that exist in your operating system. The Java 1.0 AWT programming model is also awkward and non-object-oriented. A student in one of my seminars (who had been at Sun during the creation of Java) explained why: the original AWT had been conceptualized, designed, and implemented in a month. Certainly a marvel of productivity, and also an object lesson in why design is important. Comment
The situation improved with the Java 1.1 AWT event model, which takes a much clearer, object-oriented approach, along with the addition of JavaBeans, a component programming model that is oriented toward the easy creation of visual programming environments. Java 2 finishes the transformation away from the old Java 1.0 AWT by essentially replacing everything with the Java Foundation Classes (JFC), the GUI portion of which is called “Swing.” These are a rich set of easy-to-use, easy-to-understand JavaBeans that can be dragged and dropped (as well as hand programmed) to create a GUI that you can (finally) be satisfied with. The “revision 3” rule of the software industry (a product isn’t good until revision 3) seems to hold true with programming languages as well. Comment
This chapter does not cover anything but the modern, Java 2 Swing library, and makes the reasonable assumption that Swing is the final destination GUI library for Java. If for some reason you need to use the original “old” AWT (because you’re supporting old code or you have browser limitations), you can find that introduction in the first edition of this book, downloadable at www.BruceEckel.com (also included on the CD ROM bound with this book). Comment
Early in this chapter, you’ll see how things are different when you want to create an applet vs. a regular application using Swing, and how to create programs that are both applets and applications so they can be run either inside a browser or from the command line. Almost all the GUI examples in this book will be executable as either applets or applications. Comment
Please be aware that this is not a comprehensive glossary of either all the Swing components, or all the methods for the described classes. What you see here is intended to be simple. The Swing library is vast, and the goal of this chapter is only to get you started with the essentials and comfortable with the concepts. If you need to do more, then Swing can probably give you what you want if you’re willing to do the research. Comment
I assume here that you have downloaded and installed the (free) Java library documents in HTML format from java.sun.com and will browse the javax.swing classes in that documentation to see the full details and methods of the Swing library. Because of the simplicity of the Swing design, this will often be enough information to solve your problem. There are numerous (rather thick) books dedicated solely to Swing and you’ll want to go to those if you need more depth, or if you want to modify the default Swing behavior. Comment
As you learn about Swing you’ll discover: Comment
Swing contains all the components that you expect to see in a modern UI, everything from buttons that contain pictures to trees and tables. It’s a big library, but it’s designed to have appropriate complexity for the task at hand—if something is simple, you don’t have to write much code but as you try to do more complex things, your code becomes proportionally more complex. This means an easy entry point, but you’ve got the power if you need it. Comment
Much of what you’ll like about Swing could be called “orthogonality of use.” That is, once you pick up the general ideas about the library you can apply them everywhere. Primarily because of the standard naming conventions, much of the time that I was writing these examples I could guess at the method names and get it right the first time, without looking anything up. This is certainly the hallmark of a good library design. In addition, you can generally plug components into other components and things will work correctly. Comment
For speed, all the components are “lightweight,” and Swing is written entirely in Java for portability. Comment
Keyboard navigation is automatic—you can run a Swing application without using the mouse, and this doesn’t require any extra programming. Scrolling support is effortless—you simply wrap your component in a JScrollPane as you add it to your form. Features such as tool tips typically require a single line of code to use. Comment
Swing also supports a rather radical feature called “pluggable look and feel,” which means that the appearance of the UI can be dynamically changed to suit the expectations of users working under different platforms and operating systems. It’s even possible (albeit difficult) to invent your own look and feel. Comment
One of Java’s design goals is to create applets, which are little programs that run inside a Web browser. Because they must be safe, applets are limited in what they can accomplish. However, applets are a powerful tool that support client-side programming, a major issue for the Web. Comment
Programming within an applet is so restrictive that it’s often referred to as being “inside the sandbox,” since you always have someone—that is, the Java run-time security system—watching over you. Comment
However, you can also step outside the sandbox and write regular applications rather than applets, in which case you can access the other features of your OS. We’ve been writing regular applications all along in this book, but they’ve been console applications without any graphical components. Swing can also be used to build GUI interfaces for regular applications. Comment
You can generally answer the question of what an applet is able to do by looking at what it is supposed to do: extend the functionality of a Web page in a browser. Since, as a Net surfer, you never really know if a Web page is from a friendly place or not, you want any code that it runs to be safe. So the biggest restrictions you’ll notice are probably: Comment
If you can live within the restrictions, applets have definite advantages, especially when building client/server or other networked applications: Comment
Because applets are automatically integrated with HTML, you have a built-in platform-independent documentation system to support the applet. It’s an interesting twist, since we’re used to having the documentation part of the program rather than vice versa. Comment
Libraries are often grouped according to their functionality. Some libraries, for example, are used as is, off the shelf. The standard Java library String and ArrayList classes are examples of these. Other libraries are designed specifically as building blocks to create other classes. A certain category of library is the application framework, whose goal is to help you build applications by providing a class or set of classes that produces the basic behavior that you need in every application of a particular type. Then, to customize the behavior to your own needs, you inherit from the application class and override the methods of interest. The application framework’s default control mechanism will call your overridden methods at the appropriate time. An application framework is a good example of “separating the things that change from the things that stay the same,” since it attempts to localize all the unique parts of a program in the overridden methods[68]. Comment
Applets are built using an application framework. You inherit from class JApplet and override the appropriate methods. There are a few methods that control the creation and execution of an applet on a Web page:
|
Method |
Operation |
|---|---|
|
init( ) |
Automatically called to perform first-time initialization of the applet, including component layout. You’ll always override this method. |
|
start( ) |
Called every time the applet moves into sight on the Web browser to allow the applet to start up its normal operations (especially those that are shut off by stop( )). Also called after init( ). |
|
stop( ) |
Called every time the applet moves out of sight on the Web browser to allow the applet to shut off expensive operations. Also called right before destroy( ). |
|
destroy( ) |
Called when the applet is being unloaded from the page to perform final release of resources when the applet is no longer used |
With this information you are ready to create a simple applet:
//: c13:Applet1.java
// Very simple applet.
import javax.swing.*;
import java.awt.*;
public class Applet1 extends JApplet {
public void init() {
getContentPane().add(new JLabel("Applet!"));
}
} ///:~Note that applets are not required to have a main( ). That’s all wired into the application framework; you put any startup code in init( ). Comment
In this program, the only activity is putting a text label on the applet, via the JLabel class (the old AWT appropriated the name Label as well as other names of components, so you will often see a leading “J” used with Swing components). The constructor for this class takes a String and uses it to create the label. In the above program this label is placed on the form. Comment
The init( ) method is responsible for putting all the components on the form using the add( ) method. You might think that you ought to be able to simply call add( ) by itself, and in fact that’s the way it used to be in the old AWT. However, Swing requires you to add all components to the “content pane” of a form, and so you must call getContentPane( ) as part of the add( ) process. Comment
To run this program you must place it inside a Web page and view that page inside your Java-enabled Web browser. To place an applet inside a Web page you put a special tag inside the HTML source for that Web page[69] to tell the page how to load and run the applet. Comment
This process used to be very simple, when Java itself was simple and everyone was on the same bandwagon and incorporated the same Java support inside their Web browsers. Then you might have been able to get away with a very simple bit of HTML inside your Web page, like this:
<applet code=Applet1 width=100 height=50> </applet>
Then along came the browser and language wars, and we (programmers and end users alike) lost. After awhile, JavaSoft realized that we could no longer expect browsers to support the correct flavor of Java, and the only solution was to provide some kind of add-on that would conform to a browser’s extension mechanism. By using the extension mechanism (which a browser vendor cannot disable—in an attempt to gain competitive advantage—without breaking all the third-party extensions), JavaSoft guarantees that Java cannot be shut out of the Web browser by an antagonistic vendor. Comment
With Internet Explorer, the extension mechanism is the ActiveX control, and with Netscape it is the plug-in. In your HTML code, you must provide tags to support both. Here’s what the simplest resulting HTML page looks like for Applet1:[70]
//:! c13:Applet1.html
<html><head><title>Applet1</title></head><hr>
<OBJECT
classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"
width="100" height="50" align="baseline" codebase="http://java.sun.com/products/plugin/1.2.2/jinstall-1_2_2-win.cab#Version=1,2,2,0">
<PARAM NAME="code" VALUE="Applet1.class">
<PARAM NAME="codebase" VALUE=".">
<PARAM NAME="type" VALUE="application/x-java-applet;version=1.2.2">
<COMMENT>
<EMBED type=
"application/x-java-applet;version=1.2.2"
width="200" height="200" align="baseline"
code="Applet1.class" codebase="."
pluginspage="http://java.sun.com/products/plugin/1.2/plugin-install.html">
<NOEMBED>
</COMMENT>
No Java 2 support for APPLET!!
</NOEMBED>
</EMBED>
</OBJECT>
<hr></body></html>
///:~Some of these lines were too long and had to be wrapped to fit on the page. The code in this book’s source code (on the book’s CD ROM, and downloadable from www.BruceEckel.com) will work without having to worry about correcting line wraps. Comment
The code value gives the name of the .class file where the applet resides. The width and height specify the initial size of the applet (in pixels, as before). There are other items you can place within the applet tag: a place to find other .class files on the Internet (codebase), alignment information (align), a special identifier that makes it possible for applets to communicate with each other (name), and applet parameters to provide information that the applet can retrieve. Parameters are in the form:
<param name="identifier" value = "information">
and there can be as many as you want. Comment
The source code package for this book provides an HTML page for each of the applets in this book, and thus many examples of the applet tag. You can find a full and current description of the details of placing applets in Web pages at java.sun.com. Comment
Sun’s JDK (freely downloadable from java.sun.com) contains a tool called the Appletviewer that picks the <applet> tags out of the HTML file and runs the applets without displaying the surrounding HTML text. Because the Appletviewer ignores everything but APPLET tags, you can put those tags in the Java source file as comments:
// <applet code=MyApplet width=200 height=100> // </applet>
This way, you can run “appletviewer MyApplet.java” and you don’t need to create tiny HTML files to run tests. For example, you can add the commented HTML tags to Applet1.java:
//: c13:Applet1b.java
// Embedding the applet tag for Appletviewer.
// <applet code=Applet1b width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
public class Applet1b extends JApplet {
public void init() {
getContentPane().add(new JLabel("Applet!"));
}
} ///:~Now you can invoke the applet with the command
appletviewer Applet1b.java
In this book, this form will be used for easy testing of applets. Shortly, you’ll see another coding approach which will allow you to execute applets from the command line without the Appletviewer. Comment
You can perform a simple test without any network connection by starting up your Web browser and opening the HTML file containing the applet tag. As the HTML file is loaded, the browser will discover the applet tag and go hunt for the .class file specified by the code value. Of course, it looks at the CLASSPATH to find out where to hunt, and if your .class file isn’t in the CLASSPATH then it will give an error message on the status line of the browser to the effect that it couldn’t find that .class file. Comment
When you want to try this out on your Web site things are a little more complicated. First of all, you must have a Web site, which for most people means a third-party Internet Service Provider (ISP) at a remote location. Since the applet is just a file or set of files, the ISP does not have to provide any special support for Java. You must also have a way to move the HTML files and the .class files from your site to the correct directory on the ISP machine. This is typically done with a File Transfer Protocol (FTP) program, of which there are many different types available for free or as shareware. So it would seem that all you need to do is move the files to the ISP machine with FTP, then connect to the site and HTML file using your browser; if the applet comes up and works, then everything checks out, right? Comment
Here’s where you can get fooled. If the browser on the client machine cannot locate the .class file on the server, it will hunt through the CLASSPATH on your local machine. Thus, the applet might not be loading properly from the server, but to you it looks fine during your testing process because the browser finds it on your machine. When someone else connects, however, his or her browser can’t find it. So when you’re testing, make sure you erase the relevant .class files (or .jar file) on your local machine to verify that they exist in the proper location on the server. Comment
One of the most insidious places where this happened to me is when I innocently placed an applet inside a package. After uploading the HTML file and applet, it turned out that the server path to the applet was confused because of the package name. However, my browser found it in the local CLASSPATH. So I was the only one who could properly load the applet. It took some time to discover that the package statement was the culprit. In general, you’ll want to leave the package statement out of an applet. Comment
There are times when you’d like to make a windowed program do something else other than sit on a Web page. Perhaps you’d also like it to do some of the things a “regular” application can do but still have the vaunted instant portability provided by Java. In previous chapters in this book we’ve made command-line applications, but in some operating environments (the Macintosh, for example) there isn’t a command line. So for any number of reasons you’d like to build a windowed, non-applet program using Java. This is certainly a reasonable desire. Comment
The Swing library allows you to make an application that preserves the look and feel of the underlying operating environment. If you want to build windowed applications, it makes sense to do so[71] only if you can use the latest version of Java and associated tools so you can deliver applications that won’t confound your users. If for some reason you’re forced to use an older version of Java, think hard before committing to building a significant windowed application. Comment
Often you’ll want to be able to create a class that can be invoked as either a window or an applet. This is especially convenient when you’re testing the applets, since it’s typically much faster and easier to run the resulting applet-application from the command line than it is to start up a Web browser or the Appletviewer. Comment
To create an applet that can be run from the console command line, you simply add a main( ) to your applet that builds an instance of the applet inside a JFrame.[72] As a simple example, let’s look at Applet1b.java modified to work as both an application and an applet:
//: c13:Applet1c.java
// An application and an applet.
// <applet code=Applet1c width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Applet1c extends JApplet {
public void init() {
getContentPane().add(new JLabel("Applet!"));
}
// A main() for the application:
public static void main(String[] args) {
JApplet applet = new Applet1c();
JFrame frame = new JFrame("Applet1c");
// To close the application:
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(applet);
frame.setSize(100,50);
applet.init();
applet.start();
frame.setVisible(true);
}
} ///:~main( ) is the only element added to the applet, and the rest of the applet is untouched. The applet is created and added to a JFrame so that it can be displayed. Comment
The line:
Console.setupClosing(frame);
Causes the window to be properly closed. Console comes from com.bruceeckel.swing and will be explained a little later. Comment
You can see that in main( ), the applet is explicitly initialized and started since in this case the browser isn’t available to do it for you. Of course, this doesn’t provide the full behavior of the browser, which also calls stop( ) and destroy( ), but for most situations it’s acceptable. If it’s a problem, you can force the calls yourself.[73] Comment
Notice the last line:
frame.setVisible(true);
Without this, you won’t see anything on the screen. Comment
Although the code that turns programs into both applets and applications produces valuable results, if used everywhere it becomes distracting and wastes paper. Instead, the following display framework will be used for the Swing examples in the rest of this book:
//: com:bruceeckel:swing:Console.java
// Tool for running Swing demos from the
// console, both applets and JFrames.
package com.bruceeckel.swing;
import javax.swing.*;
import java.awt.event.*;
public class Console {
// Create a title string from the class name:
public static String title(Object o) {
String t = o.getClass().toString();
// Remove the word "class":
if(t.indexOf("class") != -1)
t = t.substring(6);
return t;
}
public static void
run(JFrame frame, int width, int height) {
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(width, height);
frame.setVisible(true);
}
public static void
run(JApplet applet, int width, int height) {
JFrame frame = new JFrame(title(applet));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(applet);
frame.setSize(width, height);
applet.init();
applet.start();
frame.setVisible(true);
}
public static void
run(JPanel panel, int width, int height) {
JFrame frame = new JFrame(title(panel));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(panel);
frame.setSize(width, height);
frame.setVisible(true);
}
} ///:~This is a tool you may want to use yourself, so it’s placed in the library com.bruceeckel.swing. The Console class consists entirely of static methods. The first is used to extract the class name (using RTTI) from any object and to remove the word “class,” which is typically prepended by getClass( ). This uses the String methods indexOf( ) to determine whether the word “class” is there, and substring( ) to produce the new string without “class” or the trailing space. This name is used to label the window that is displayed by the run( ) methods. Comment
setupClosing( ) is used to hide the code that causes a JFrame to exit a program when that JFrame is closed. The default behavior is to do nothing, so if you don’t call setupClosing( ) or write the equivalent code for your JFrame, the application won’t close. The reason this code is hidden rather than placing it directly in the subsequent run( ) methods is partly because it allows you to use the method by itself when what you want to do is more complicated than what run( ) provides. However, it also isolates a change factor: Java 2 has two ways of causing certain types of windows to close. In JDK 1.2, the solution is to create a new WindowAdapter class and implement windowClosing( ), as seen above (the meaning of this will be fully explained later in this chapter). However, during the creation of JDK 1.3 the library designers observed that you typically need to close windows whenever you’re creating a non-applet, and so they added the setDefaultCloseOperation( ) to JFrame and JDialog. From the standpoint of writing code, the new method is much nicer to use but this book was written while there was still no JDK 1.3 implemented on Linux and other platforms, so in the interest of cross-version portability the change was isolated inside setupClosing( ). Comment
The run( ) method is overloaded to work with JApplets, JPanels, and JFrames. Note that only if it’s a JApplet are init( ) and start( ) called. Comment
Now any applet can be run from the console by creating a main( ) containing a line like this:
Console.run(new MyClass(), 500, 300);
in which the last two arguments are the display width and height. Here’s Applet1c.java modified to use Console:
//: c13:Applet1d.java
// Console runs applets from the command line.
// <applet code=Applet1d width=100 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Applet1d extends JApplet {
public void init() {
getContentPane().add(new JLabel("Applet!"));
}
public static void main(String[] args) {
Console.run(new Applet1d(), 100, 50);
}
} ///:~This allows the elimination of repeated code while providing the greatest flexibility in running the examples. Comment
If you’re using Windows, you can simplify the process of running a command-line Java program by configuring the Windows Explorer—the file browser in Windows, not the Internet Explorer—so that you can simply double-click on a .class file to execute it. There are several steps in this process. Comment
First, download and install the Perl programming language from www.Perl.org. You’ll find the instructions and language documentation on that site. Comment
Next, create the following script without the first and last lines (this script is part of this book’s source-code package):
//:! c13:RunJava.bat @rem = '--*-Perl-*-- @echo off perl -x -S "%0" %1 %2 %3 %4 %5 %6 %7 %8 %9 goto endofperl @rem '; #!perl $file = $ARGV[0]; $file =~ s/(.*)\..*/\1/; $file =~ s/(.*\\)*(.*)/$+/; `java $file`; __END__ :endofperl ///:~
Now, open the Windows Explorer, select “View,” “Folder Options,” then click on the “File Types” tab. Press the “New Type” button. For “Description of Type” enter “Java class file.” For “Associated Extension,” enter “class.” Under “Actions” press the “New” button. For “Action” enter “Open,” and for “Application used to perform action” enter a line like this:
"c:\aaa\Perl\RunJava.bat" "%L"
You must customize the path before “RunJava.bat” to conform to the location where you placed the batch file. Comment
Once you perform this installation, you may run any Java program by simply double-clicking on the .class file containing a main( ). Comment
Making a button is quite simple: you just call the JButton constructor with the label you want on the button. You’ll see later that you can do fancier things, like putting graphic images on buttons. Comment
Usually you’ll want to create a field for the button inside your class so that you can refer to it later. Comment
The JButton is a component—its own little window—that will automatically get repainted as part of an update. This means that you don’t explicitly paint a button or any other kind of control; you simply place them on the form and let them automatically take care of painting themselves. So to place a button on a form, you do it inside init( ):
//: c13:Button1.java
// Putting buttons on an applet.
// <applet code=Button1 width=200 height=50>
// </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Button1 extends JApplet {
JButton
b1 = new JButton("Button 1"),
b2 = new JButton("Button 2");
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b1);
cp.add(b2);
}
public static void main(String[] args) {
Console.run(new Button1(), 200, 50);
}
} ///:~Something new has been added here: before any elements are placed on the content pane, it is given a new “layout manager,” of type FlowLayout. The layout manager is the way that the pane implicitly decides where to place the control on the form. The normal behavior of an applet is to use the BorderLayout, but that won’t work here because (as you will learn later in this chapter when controlling the layout of a form is examined in more detail) it defaults to covering each control entirely with every new one that is added. However, FlowLayout causes the controls to flow evenly onto the form, left to right and top to bottom. Comment
You’ll notice that if you compile and run the applet above, nothing happens when you press the buttons. This is where you must step in and write some code to determine what will happen. The basis of event-driven programming, which comprises a lot of what a GUI is about, is tying events to code that responds to those events. Comment
The way that this is accomplished in Swing is by cleanly separating the interface (the graphical components) and the implementation (the code that you want to run when an event happens to a component). Each Swing component can report all the events that might happen to it, and it can report each kind of event individually. So if you’re not interested in, for example, whether the mouse is being moved over your button, you don’t register your interest in that event. It’s a very straightforward and elegant way to handle event-driven programming, and once you understand the basic concepts you can easily use Swing components that you haven’t seen before—in fact, this model extends to anything that can be classified as a JavaBean (which you’ll learn about later in the chapter). Comment
At first, we will just focus on the main event of interest for the components being used. In the case of a JButton, this “event of interest” is that the button is pressed. To register your interest in when a button is pressed, you call the JButton’s addActionListener( ) method. This method expects an argument that is an object that implements the ActionListener interface, which contains a single method called actionPerformed( ). So all you have to do to attach code to a JButton is to implement the ActionListener interface in a class, and register an object of that class with the JButton via addActionListener( ). The method will be called when the button is pressed (this is normally referred to as a callback). Comment
But what should the result of pressing that button be? We’d like to see something change on the screen, so a new Swing component will be introduced: the JTextField. This is a place where text can be typed, or in this case modified by the program. Although there are a number of ways to create a JTextField, the simplest is just to tell the constructor how wide you want that field to be. Once the JTextField is placed on the form, you can modify its contents by using the setText( ) method (there are many other methods in JTextField, but you must look these up in the HTML documentation for the JDK from java.sun.com). Here is what it looks like:
//: c13:Button2.java
// Responding to button presses.
// <applet code=Button2 width=200 height=75>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Button2 extends JApplet {
JButton
b1 = new JButton("Button 1"),
b2 = new JButton("Button 2");
JTextField txt = new JTextField(10);
class BL implements ActionListener {
public void actionPerformed(ActionEvent e){
String name =
((JButton)e.getSource()).getText();
txt.setText(name);
}
}
BL al = new BL();
public void init() {
b1.addActionListener(al);
b2.addActionListener(al);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b1);
cp.add(b2);
cp.add(txt);
}
public static void main(String[] args) {
Console.run(new Button2(), 200, 75);
}
} ///:~Creating a JTextField and placing it on the canvas takes the same steps as for JButtons, or for any Swing component. The difference in the above program is in the creation of the aforementioned ActionListener class BL. The argument to actionPerformed( ) is of type ActionEvent, which contains all the information about the event and where it came from. In this case, I wanted to describe the button that was pressed: getSource( ) produces the object where the event originated, and I assumed that is a JButton. getText( ) returns the text that’s on the button, and this is placed in the JTextField to prove that the code was actually called when the button was pressed. Comment
In init( ), addActionListener( ) is used to register the BL object with both the buttons. Comment
It is often more convenient to code the ActionListener as an anonymous inner class, especially since you tend to only use a single instance of each listener class. Button2.java can be modified to use an anonymous inner class as follows: Comment
//: c13:Button2b.java
// Using anonymous inner classes.
// <applet code=Button2b width=200 height=75>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Button2b extends JApplet {
JButton
b1 = new JButton("Button 1"),
b2 = new JButton("Button 2");
JTextField txt = new JTextField(10);
ActionListener al = new ActionListener() {
public void actionPerformed(ActionEvent e){
String name =
((JButton)e.getSource()).getText();
txt.setText(name);
}
};
public void init() {
b1.addActionListener(al);
b2.addActionListener(al);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b1);
cp.add(b2);
cp.add(txt);
}
public static void main(String[] args) {
Console.run(new Button2b(), 200, 75);
}
} ///:~The approach of using an anonymous inner class will be preferred (when possible) for the examples in this book. Comment
A JTextArea is like a JTextField except that it can have multiple lines and has more functionality. A particularly useful method is append( ); with this you can easily pour output into the JTextArea, thus making a Swing program an improvement (since you can scroll backward) over what has been accomplished thus far using command-line programs that print to standard output. As an example, the following program fills a JTextArea with the output from the geography generator in Chapter 9:
//: c13:TextArea.java
// Using the JTextArea control.
// <applet code=TextArea width=475 height=425>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.util.*;
import com.bruceeckel.swing.*;
import com.bruceeckel.util.*;
public class TextArea extends JApplet {
JButton
b = new JButton("Add Data"),
c = new JButton("Clear Data");
JTextArea t = new JTextArea(20, 40);
Map m = new HashMap();
public void init() {
// Use up all the data:
Collections2.fill(m,
Collections2.geography,
CountryCapitals.pairs.length);
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
for(Iterator it= m.entrySet().iterator();
it.hasNext();){
Map.Entry me = (Map.Entry)(it.next());
t.append(me.getKey() + ": "
+ me.getValue() + "\n");
}
}
});
c.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
t.setText("");
}
});
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(new JScrollPane(t));
cp.add(b);
cp.add(c);
}
public static void main(String[] args) {
Console.run(new TextArea(), 475, 425);
}
} ///:~In init( ), the Map is filled with all the countries and their capitals. Note that for both buttons the ActionListener is created and added without defining an intermediate variable, since you never need to refer to that listener again during the program. The “Add Data” button formats and appends all the data, while the “Clear Data” button uses setText( ) to remove all the text from the JTextArea. Comment
As the JTextArea is added to the applet, it is wrapped in a JScrollPane, to control scrolling when too much text is placed on the screen. That’s all you must do in order to produce full scrolling capabilities. Having tried to figure out how to do the equivalent in some other GUI programming environments, I am very impressed with the simplicity and good design of components like JScrollPane. Comment
The way that you place components on a form in Java is probably different from any other GUI system you’ve used. First, it’s all code; there are no “resources” that control placement of components. Second, the way components are placed on a form is controlled not by absolute positioning but by a “layout manager” that decides how the components lie based on the order that you add( ) them. The size, shape, and placement of components will be remarkably different from one layout manager to another. In addition, the layout managers adapt to the dimensions of your applet or application window, so if the window dimension is changed, the size, shape, and placement of the components can change in response. Comment
JApplet, JFrame JWindow, and JDialog can all produce a Container with getContentPane( ) that can contain and display Components. In Container, there’s a method called setLayout( ) that allows you to choose a different layout manager. Other classes, such as JPanel, contain and display components directly and so you also set the layout manager directly, without using the content pane. Comment
In this section we’ll explore the various layout managers by placing buttons in them (since that’s the simplest thing to do). There won’t be any capturing of button events since these examples are just intended to show how the buttons are laid out. Comment
The applet uses a default layout scheme: the BorderLayout (a number of the previous example have changed the layout manager to FlowLayout). Without any other instruction, this takes whatever you add( ) to it and places it in the center, stretching the object all the way out to the edges. Comment
However, there’s more to the BorderLayout. This layout manager has the concept of four border regions and a center area. When you add something to a panel that’s using a BorderLayout you can use the overloaded add( ) method that takes a constant value as its first argument. This value can be any of the following:
BorderLayout. NORTH (top)
BorderLayout. SOUTH (bottom)
BorderLayout. EAST (right)
BorderLayout. WEST (left)
BorderLayout.CENTER (fill the middle, up to the other components or to the edges) Comment
If you don’t specify an area to place the object, it defaults to CENTER. Comment
Here’s a simple example. The default layout is used, since JApplet defaults to BorderLayout:
//: c13:BorderLayout1.java
// Demonstrates BorderLayout.
// <applet code=BorderLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class BorderLayout1 extends JApplet {
public void init() {
Container cp = getContentPane();
cp.add(BorderLayout.NORTH,
new JButton("North"));
cp.add(BorderLayout.SOUTH,
new JButton("South"));
cp.add(BorderLayout.EAST,
new JButton("East"));
cp.add(BorderLayout.WEST,
new JButton("West"));
cp.add(BorderLayout.CENTER,
new JButton("Center"));
}
public static void main(String[] args) {
Console.run(new BorderLayout1(), 300, 250);
}
} ///:~For every placement but CENTER, the element that you add is compressed to fit in the smallest amount of space along one dimension while it is stretched to the maximum along the other dimension. CENTER, however, spreads out in both dimensions to occupy the middle. Comment
This simply “flows” the components onto the form, from left to right until the top space is full, then moves down a row and continues flowing. Comment
Here’s an example that sets the layout manager to FlowLayout and then places buttons on the form. You’ll notice that with FlowLayout the components take on their “natural” size. A JButton, for example, will be the size of its string. Comment
//: c13:FlowLayout1.java
// Demonstrates FlowLayout.
// <applet code=FlowLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class FlowLayout1 extends JApplet {
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
for(int i = 0; i < 20; i++)
cp.add(new JButton("Button " + i));
}
public static void main(String[] args) {
Console.run(new FlowLayout1(), 300, 250);
}
} ///:~All components will be compacted to their smallest size in a FlowLayout, so you might get a little bit of surprising behavior. For example, because a JLabel will be the size of its string, attempting to right-justify its text yields an unchanged display when using FlowLayout. Comment
A GridLayout allows you to build a table of components, and as you add them they are placed left-to-right and top-to-bottom in the grid. In the constructor you specify the number of rows and columns that you need and these are laid out in equal proportions. Comment
//: c13:GridLayout1.java
// Demonstrates GridLayout.
// <applet code=GridLayout1
// width=300 height=250> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class GridLayout1 extends JApplet {
public void init() {
Container cp = getContentPane();
cp.setLayout(new GridLayout(7,3));
for(int i = 0; i < 20; i++)
cp.add(new JButton("Button " + i));
}
public static void main(String[] args) {
Console.run(new GridLayout1(), 300, 250);
}
} ///:~In this case there are 21 slots but only 20 buttons. The last slot is left empty because no “balancing” goes on with a GridLayout. Comment
The GridBagLayout provides you with tremendous control in deciding exactly how the regions of your window will lay themselves out and reformat themselves when the window is resized. However, it’s also the most complicated layout manager, and quite difficult to understand. It is intended primarily for automatic code generation by a GUI builder (good GUI builders will use GridBagLayout instead of absolute placement). If your design is so complicated that you feel you need to use GridBagLayout, then you should be using a GUI builder tool to generate that design. If you feel you must know the intricate details, I’ll refer you to Core Java 2 by Horstmann & Cornell (Prentice-Hall, 1999), or a dedicated Swing book, as a starting point. Comment
It is also possible to set the absolute position of the graphical components in this way:
Some GUI builders use this approach extensively, but this is usually not the best way to generate code. More useful GUI builders will use GridBagLayout instead. Comment
Because people had so much trouble understanding and working with GridBagLayout, Swing also includes the BoxLayout, which gives you many of the benefits of GridBagLayout without the complexity, so you can often use it when you need to do hand-coded layouts (again, if your design becomes too complex, use a GUI builder that generates GridBagLayouts for you). BoxLayout allows you to control the placement of components either vertically or horizontally, and to control the space between the components using something called “struts and glue.” First, let’s see how to use the BoxLayout directly, in the same way that the other layout managers have been demonstrated:
//: c13:BoxLayout1.java
// Vertical and horizontal BoxLayouts.
// <applet code=BoxLayout1
// width=450 height=200> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class BoxLayout1 extends JApplet {
public void init() {
JPanel jpv = new JPanel();
jpv.setLayout(
new BoxLayout(jpv, BoxLayout.Y_AXIS));
for(int i = 0; i < 5; i++)
jpv.add(new JButton("" + i));
JPanel jph = new JPanel();
jph.setLayout(
new BoxLayout(jph, BoxLayout.X_AXIS));
for(int i = 0; i < 5; i++)
jph.add(new JButton("" + i));
Container cp = getContentPane();
cp.add(BorderLayout.EAST, jpv);
cp.add(BorderLayout.SOUTH, jph);
}
public static void main(String[] args) {
Console.run(new BoxLayout1(), 450, 200);
}
} ///:~The constructor for BoxLayout is a bit different than the other layout managers—you provide the Container that is to be controlled by the BoxLayout as the first argument, and the direction of the layout as the second argument. Comment
To simplify matters, there’s a special container called Box that uses BoxLayout as its native manager. The following example lays out components horizontally and vertically using Box, which has two static methods to create boxes with vertical and horizontal alignment:
//: c13:Box1.java
// Vertical and horizontal BoxLayouts.
// <applet code=Box1
// width=450 height=200> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Box1 extends JApplet {
public void init() {
Box bv = Box.createVerticalBox();
for(int i = 0; i < 5; i++)
bv.add(new JButton("" + i));
Box bh = Box.createHorizontalBox();
for(int i = 0; i < 5; i++)
bh.add(new JButton("" + i));
Container cp = getContentPane();
cp.add(BorderLayout.EAST, bv);
cp.add(BorderLayout.SOUTH, bh);
}
public static void main(String[] args) {
Console.run(new Box1(), 450, 200);
}
} ///:~Once you have a Box, you pass it as a second argument when adding components to the content pane. Comment
Struts add space between components, measured in pixels. To use a strut, you simply add it in between the addition of the components that you want spaced apart:
//: c13:Box2.java
// Adding struts.
// <applet code=Box2
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Box2 extends JApplet {
public void init() {
Box bv = Box.createVerticalBox();
for(int i = 0; i < 5; i++) {
bv.add(new JButton("" + i));
bv.add(Box.createVerticalStrut(i*10));
}
Box bh = Box.createHorizontalBox();
for(int i = 0; i < 5; i++) {
bh.add(new JButton("" + i));
bh.add(Box.createHorizontalStrut(i*10));
}
Container cp = getContentPane();
cp.add(BorderLayout.EAST, bv);
cp.add(BorderLayout.SOUTH, bh);
}
public static void main(String[] args) {
Console.run(new Box2(), 450, 300);
}
} ///:~Struts separate components by a fixed amount, but glue is the opposite: it separates components by as much as possible. Thus it’s more of a “spring” than “glue” (and the design on which this was based was called “springs and struts” so the choice of the term is a bit mysterious). Comment
//: c13:Box3.java
// Using Glue.
// <applet code=Box3
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Box3 extends JApplet {
public void init() {
Box bv = Box.createVerticalBox();
bv.add(new JLabel("Hello"));
bv.add(Box.createVerticalGlue());
bv.add(new JLabel("Applet"));
bv.add(Box.createVerticalGlue());
bv.add(new JLabel("World"));
Box bh = Box.createHorizontalBox();
bh.add(new JLabel("Hello"));
bh.add(Box.createHorizontalGlue());
bh.add(new JLabel("Applet"));
bh.add(Box.createHorizontalGlue());
bh.add(new JLabel("World"));
bv.add(Box.createVerticalGlue());
bv.add(bh);
bv.add(Box.createVerticalGlue());
getContentPane().add(bv);
}
public static void main(String[] args) {
Console.run(new Box3(), 450, 300);
}
} ///:~A strut works in one direction, but a rigid area fixes the spacing between components in both directions:
//: c13:Box4.java
// Rigid Areas are like pairs of struts.
// <applet code=Box4
// width=450 height=300> </applet>
import javax.swing.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class Box4 extends JApplet {
public void init() {
Box bv = Box.createVerticalBox();
bv.add(new JButton("Top"));
bv.add(Box.createRigidArea(
new Dimension(120, 90)));
bv.add(new JButton("Bottom"));
Box bh = Box.createHorizontalBox();
bh.add(new JButton("Left"));
bh.add(Box.createRigidArea(
new Dimension(160, 80)));
bh.add(new JButton("Right"));
bv.add(bh);
getContentPane().add(bv);
}
public static void main(String[] args) {
Console.run(new Box4(), 450, 300);
}
} ///:~You should be aware that rigid areas are a bit controversial. Since they use absolute values, some people feel that they cause more trouble than they are worth. Comment
Swing is powerful; it can get a lot done with a few lines of code. The examples shown in this book are reasonably simple, and for learning purposes it makes sense to write them by hand. You can actually accomplish quite a bit by combining simple layouts. At some point, however, it stops making sense to hand-code GUI forms—it becomes too complicated and is not a good use of your programming time. The Java and Swing designers oriented the language and libraries to support GUI building tools, which have been created for the express purpose of making your programming experience easier. As long as you understand what’s going on with layouts and how to deal with the events (described next), it’s not particularly important that you actually know the details of how to lay out components by hand—let the appropriate tool do that for you (Java is, after all, designed to increase programmer productivity). Comment
In the Swing event model a component can initiate (“fire”) an event. Each type of event is represented by a distinct class. When an event is fired, it is received by one or more “listeners,” which act on that event. Thus, the source of an event and the place where the event is handled can be separate. Since you typically use Swing components as they are, but need to write code that is called when the components receive an event, this is an excellent example of the separation of interface and implementation. Comment
Each event listener is an object of a class that implements a particular type of listener interface. So as a programmer, all you do is create a listener object and register it with the component that’s firing the event. This registration is performed by calling an addXXXListener( ) method in the event-firing component, in which “XXX” represents the type of event listened for. You can easily know what types of events can be handled by noticing the names of the “addListener” methods, and if you try to listen for the wrong events you’ll discover your mistake at compile-time. You’ll see later in the chapter that JavaBeans also use the names of the “addListener” methods to determine what events a Bean can handle. Comment
All of your event logic, then, will go inside a listener class. When you create a listener class, the sole restriction is that it must implement the appropriate interface. You can create a global listener class, but this is a situation in which inner classes tend to be quite useful, not only because they provide a logical grouping of your listener classes inside the UI or business logic classes they are serving, but because (as you shall see later) the fact that an inner class object keeps a reference to its parent object provides a nice way to call across class and subsystem boundaries. Comment
All the examples so far in this chapter have been using the Swing event model, but the remainder of this section will fill out the details of that model. Comment
All Swing components include addXXXListener( ) and removeXXXListener( ) methods so that the appropriate types of listeners can be added and removed from each component. You’ll notice that the “XXX” in each case also represents the argument for the method, for example: addMyListener(MyListener m). The following table includes the basic associated events, listeners, and methods, along with the basic components that support those particular events by providing the addXXXListener( ) and removeXXXListener( ) methods. You should keep in mind that the event model is designed to be extensible, so you may encounter other events and listener types that are not covered in this table.Comment
|
Components supporting this event |
|
|---|---|
|
ActionEvent |
JButton, JList, JTextField, JMenuItem and its derivatives including JCheckBoxMenuItem, JMenu, and JpopupMenu. |
|
AdjustmentEvent |
JScrollbar |
|
ComponentEvent |
*Component and its derivatives, including JButton, JCanvas, JCheckBox, JComboBox, Container, JPanel, JApplet, JScrollPane, Window, JDialog, JFileDialog, JFrame, JLabel, JList, JScrollbar, JTextArea, and JTextField. |
|
ContainerEvent |
Container and its derivatives, including JPanel, JApplet, JScrollPane, Window, JDialog, JFileDialog, and JFrame. |
|
FocusEvent |
Component and derivatives*. |
|
KeyEvent |
Component and derivatives*. |
|
MouseEvent (for both clicks and motion) |
Component and derivatives*. |
|
MouseEvent[74] (for both clicks and motion) |
Component and derivatives*. |
|
WindowEvent |
Window and its derivatives, including JDialog, JFileDialog, and JFrame. |
|
ItemEvent |
JCheckBox, JCheckBoxMenuItem, JComboBox, JList, and anything that implements the ItemSelectable interface. |
|
TextEvent |
Anything derived from JTextComponent, including JTextArea and JTextField. |
You can see that each type of component supports only certain types of events. It turns out to be rather difficult to look up all the events supported by each component. A simpler approach is to modify the ShowMethodsClean.java program from Chapter 12 so that it displays all the event listeners supported by any Swing component that you enter.Comment
Chapter 12 introduced reflection and used that feature to look up methods for a particular class—either the entire list of methods or a subset of those whose names match a keyword that you provide. The magic of this is that it can automatically show you all the methods for a class without forcing you to walk up the inheritance hierarchy examining the base classes at each level. Thus, it provides a valuable timesaving tool for programming: because the names of most Java methods are made nicely verbose and descriptive, you can search for the method names that contain a particular word of interest. When you find what you think you’re looking for, check the online documentation. Comment
However, by Chapter 12 you hadn’t seen Swing, so the tool in that chapter was developed as a command-line application. Here is the more useful GUI version, specialized to look for the “addListener” methods in Swing components:
//: c13:ShowAddListeners.java
// Display the "addXXXListener" methods of any
// Swing class.
// <applet code = ShowAddListeners
// width=500 height=400></applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;
import java.lang.reflect.*;
import java.io.*;
import com.bruceeckel.swing.*;
import com.bruceeckel.util.*;
public class ShowAddListeners extends JApplet {
Class cl;
Method[] m;
String[] n = new String[0];
JTextField name = new JTextField(25);
JTextArea results = new JTextArea(40, 65);
class NameL implements ActionListener {
public void actionPerformed(ActionEvent e) {
String nm = name.getText().trim();
if(nm.length() == 0) {
results.setText("No match");
n = new String[0];
return;
}
try {
cl = Class.forName("javax.swing." + nm);
} catch(ClassNotFoundException ex) {
results.setText("No match");
return;
}
m = cl.getMethods();
// Convert to an array of Strings:
n = new String[m.length];
for(int i = 0; i < m.length; i++)
n[i] = m[i].toString();
reDisplay();
}
}
void reDisplay() {
// Create the result set:
String[] rs = new String[n.length];
int j = 0;
for (int i = 0; i < n.length; i++)
if(n[i].indexOf("add") != -1 &&
n[i].indexOf("Listener") != -1)
rs[j++] =
n[i].substring(n[i].indexOf("add"));
results.setText("");
for (int i = 0; i < j; i++)
results.append(
StripQualifiers.strip(rs[i]) + "\n");
}
public void init() {
name.addActionListener(new NameL());
JPanel top = new JPanel();
top.add(new JLabel(
"Swing class name (press ENTER):"));
top.add(name);
Container cp = getContentPane();
cp.add(BorderLayout.NORTH, top);
cp.add(new JScrollPane(results));
}
public static void main(String[] args) {
Console.run(new ShowAddListeners(), 500,400);
}
} ///:~The StripQualifiers class defined in Chapter 12 is reused here by importing the com.bruceeckel.util library. Comment
The GUI contains a JTextField name in which you can enter the Swing class name you want to look up. The results are displayed in a JTextArea. Comment
You’ll notice that there are no buttons or other components by which to indicate that you want the search to begin. That’s because the JTextField is monitored by an ActionListener. Whenever you make a change and press ENTER, the list is immediately updated. If the text isn’t empty, it is used inside Class.forName( ) to try to look up the class. If the name is incorrect, Class.forName( ) will fail, which means that it throws an exception. This is trapped and the JTextArea is set to “No match.” But if you type in a correct name (capitalization counts), Class.forName( ) is successful and getMethods( ) will return an array of Method objects. Each of the objects in the array is turned into a String via toString( ) (this produces the complete method signature) and added to n, a String array. The array n is a member of class ShowAddListeners and is used in updating the display whenever reDisplay( ) is called. Comment
reDisplay( ) creates an array of String called rs (for “result set”). The result set is conditionally copied from the Strings in n that contain “add” and “Listener.” indexOf( ) and substring( ) are then used to remove the qualifiers like public, static, etc. Finally, StripQualifiers.strip( ) removes the extra name qualifiers. Comment
This program is a convenient way to investigate the capabilities of a Swing component. Once you know which events a particular component supports, you don’t need to look anything up to react to that event. You simply:
Here are some of the listener interfaces:
|
Methods in interface |
|
|---|---|
|
ActionListener |
actionPerformed(ActionEvent) |
|
AdjustmentListener |
adjustmentValueChanged( |
|
ComponentListener |
componentHidden(ComponentEvent) |
|
ContainerListener |
componentAdded(ContainerEvent) |
|
FocusListener |
focusGained(FocusEvent) |
|
KeyListener |
keyPressed(KeyEvent) |
|
MouseListener |
mouseClicked(MouseEvent) |
|
MouseMotionListener |
mouseDragged(MouseEvent) |
|
WindowListener |
windowOpened(WindowEvent) |
|
ItemListener |
itemStateChanged(ItemEvent) |
This is not an exhaustive listing, partly because the event model allows you to create your own event types and associated listeners. Thus, you’ll regularly come across libraries that have invented their own events, and the knowledge gained in this chapter will allow you to figure out how to use these events.Comment
In the table above, you can see that some listener interfaces have only one method. These are trivial to implement since you’ll implement them only when you want to write that particular method. However, the listener interfaces that have multiple methods can be less pleasant to use. For example, something you must always do when creating an application is provide a WindowListener to the JFrame so that when you get the windowClosing( ) event you can call System.exit( ) to exit the application. But since WindowListener is an interface, you must implement all of the other methods even if they don’t do anything. This can be annoying. Comment
To solve the problem, some (but not all) of the listener interfaces that have more than one method are provided with adapters, the names of which you can see in the table above. Each adapter provides default empty methods for each of the interface methods. Then all you need to do is inherit from the adapter and override only the methods you need to change. For example, the typical WindowListener you’ll use looks like this (remember that this has been wrapped inside the Console class in com.bruceeckel.swing):
class MyWindowListener extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
}The whole point of the adapters is to make the creation of listener classes easy. Comment
There is a downside to adapters, however, in the form of a pitfall. Suppose you write a WindowAdapter like the one above:
class MyWindowListener extends WindowAdapter {
public void WindowClosing(WindowEvent e) {
System.exit(0);
}
}This doesn’t work, but it will drive you crazy trying to figure out why, since everything will compile and run fine—except that closing the window won’t exit the program. Can you see the problem? It’s in the name of the method: WindowClosing( ) instead of windowClosing( ). A simple slip in capitalization results in the addition of a completely new method. However, this is not the method that’s called when the window is closing, so you don’t get the desired results. Despite the inconvenience, an interface will guarantee that the methods are properly implemented. Comment
To prove to yourself that these events are in fact being fired, and as an interesting experiment, it’s worth creating an applet that tracks extra behavior in a JButton (other than just whether it’s pressed or not). This example also shows you how to inherit your own button object because that’s what is used as the target of all the events of interest. To do so, you can just inherit from JButton.[75] Comment
The MyButton class is an inner class of TrackEvent, so MyButton can reach into the parent window and manipulate its text fields, which is what’s necessary to be able to write the status information into the fields of the parent. Of course this is a limited solution, since myButton can be used only in conjunction with TrackEvent. This kind of code is sometimes called “highly coupled”:
//: c13:TrackEvent.java
// Show events as they happen.
// <applet code=TrackEvent
// width=700 height=500></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;
public class TrackEvent extends JApplet {
HashMap h = new HashMap();
String[] event = {
"focusGained", "focusLost", "keyPressed",
"keyReleased", "keyTyped", "mouseClicked",
"mouseEntered", "mouseExited","mousePressed",
"mouseReleased", "mouseDragged", "mouseMoved"
};
MyButton
b1 = new MyButton(Color.blue, "test1"),
b2 = new MyButton(Color.red, "test2");
class MyButton extends JButton {
void report(String field, String msg) {
((JTextField)h.get(field)).setText(msg);
}
FocusListener fl = new FocusListener() {
public void focusGained(FocusEvent e) {
report("focusGained", e.paramString());
}
public void focusLost(FocusEvent e) {
report("focusLost", e.paramString());
}
};
KeyListener kl = new KeyListener() {
public void keyPressed(KeyEvent e) {
report("keyPressed", e.paramString());
}
public void keyReleased(KeyEvent e) {
report("keyReleased", e.paramString());
}
public void keyTyped(KeyEvent e) {
report("keyTyped", e.paramString());
}
};
MouseListener ml = new MouseListener() {
public void mouseClicked(MouseEvent e) {
report("mouseClicked", e.paramString());
}
public void mouseEntered(MouseEvent e) {
report("mouseEntered", e.paramString());
}
public void mouseExited(MouseEvent e) {
report("mouseExited", e.paramString());
}
public void mousePressed(MouseEvent e) {
report("mousePressed", e.paramString());
}
public void mouseReleased(MouseEvent e) {
report("mouseReleased", e.paramString());
}
};
MouseMotionListener mml =
new MouseMotionListener() {
public void mouseDragged(MouseEvent e) {
report("mouseDragged", e.paramString());
}
public void mouseMoved(MouseEvent e) {
report("mouseMoved", e.paramString());
}
};
public MyButton(Color color, String label) {
super(label);
setBackground(color);
addFocusListener(fl);
addKeyListener(kl);
addMouseListener(ml);
addMouseMotionListener(mml);
}
}
public void init() {
Container c = getContentPane();
c.setLayout(new GridLayout(event.length+1,2));
for(int i = 0; i < event.length; i++) {
JTextField t = new JTextField();
t.setEditable(false);
c.add(new JLabel(event[i], JLabel.RIGHT));
c.add(t);
h.put(event[i], t);
}
c.add(b1);
c.add(b2);
}
public static void main(String[] args) {
Console.run(new TrackEvent(), 700, 500);
}
} ///:~In the MyButton constructor, the button’s color is set with a call to SetBackground( ). The listeners are all installed with simple method calls. Comment
The TrackEvent class contains a HashMap to hold the strings representing the type of event and JTextFields where information about that event is held. Of course, these could have been created statically rather than putting them in a HashMap, but I think you’ll agree that it’s a lot easier to use and change. In particular, if you need to add or remove a new type of event in TrackEvent, you simply add or remove a string in the event array—everything else happens automatically. Comment
When report( ) is called it is given the name of the event and the parameter string from the event. It uses the HashMap h in the outer class to look up the actual JTextField associated with that event name, and then places the parameter string into that field. Comment
This example is fun to play with since you can really see what’s going on with the events in your program. Comment
Now that you understand layout managers and the event model, you’re ready to see how Swing components can be used. This section is a nonexhaustive tour of the Swing components and features that you’ll probably use most of the time. Each example is intended to be reasonably small so that you can easily lift the code and use it in your own programs. Comment
You can easily see what each of these examples looks like while running by viewing the HTML pages in the downloadable source code for this chapter. Comment
Keep in mind:
Swing includes a number of different types of buttons. All buttons, check boxes, radio buttons, and even menu items are inherited from AbstractButton (which, since menu items are included, would probably have been better named “AbstractChooser” or something equally general). You’ll see the use of menu items shortly, but the following example shows the various types of buttons available: Comment
//: c13:Buttons.java
// Various Swing buttons.
// <applet code=Buttons
// width=350 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.plaf.basic.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;
public class Buttons extends JApplet {
JButton jb = new JButton("JButton");
BasicArrowButton
up = new BasicArrowButton(
BasicArrowButton.NORTH),
down = new BasicArrowButton(
BasicArrowButton.SOUTH),
right = new BasicArrowButton(
BasicArrowButton.EAST),
left = new BasicArrowButton(
BasicArrowButton.WEST);
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(jb);
cp.add(new JToggleButton("JToggleButton"));
cp.add(new JCheckBox("JCheckBox"));
cp.add(new JRadioButton("JRadioButton"));
JPanel jp = new JPanel();
jp.setBorder(new TitledBorder("Directions"));
jp.add(up);
jp.add(down);
jp.add(left);
jp.add(right);
cp.add(jp);
}
public static void main(String[] args) {
Console.run(new Buttons(), 350, 100);
}
} ///:~This begins with the BasicArrowButton from javax.swing.plaf.basic, then continues with the various specific types of buttons. When you run the example, you’ll see that the toggle button holds its last position, in or out. But the check boxes and radio buttons behave identically to each other, just clicking on or off (they are inherited from JToggleButton). Comment
If you want radio buttons to behave in an “exclusive or” fashion, you must add them to a “button group.” But, as the example below demonstrates, any AbstractButton can be added to a ButtonGroup. Comment
To avoid repeating a lot of code, this example uses reflection to generate the groups of different types of buttons. This is seen in makeBPanel( ), which creates a button group and a JPanel. The second argument to makeBPanel( ) is an array of String. For each String, a button of the class represented by the first argument is added to the JPanel:
//: c13:ButtonGroups.java
// Uses reflection to create groups
// of different types of AbstractButton.
// <applet code=ButtonGroups
// width=500 height=300></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import java.lang.reflect.*;
import com.bruceeckel.swing.*;
public class ButtonGroups extends JApplet {
static String[] ids = {
"June", "Ward", "Beaver",
"Wally", "Eddie", "Lumpy",
};
static JPanel
makeBPanel(Class bClass, String[] ids) {
ButtonGroup bg = new ButtonGroup();
JPanel jp = new JPanel();
String title = bClass.getName();
title = title.substring(
title.lastIndexOf('.') + 1);
jp.setBorder(new TitledBorder(title));
for(int i = 0; i < ids.length; i++) {
AbstractButton ab = new JButton("failed");
try {
// Get the dynamic constructor method
// that takes a String argument:
Constructor ctor = bClass.getConstructor(
new Class[] { String.class });
// Create a new object:
ab = (AbstractButton)ctor.newInstance(
new Object[]{ids[i]});
} catch(Exception ex) {
System.err.println("can't create " +
bClass);
}
bg.add(ab);
jp.add(ab);
}
return jp;
}
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(makeBPanel(JButton.class, ids));
cp.add(makeBPanel(JToggleButton.class, ids));
cp.add(makeBPanel(JCheckBox.class, ids));
cp.add(makeBPanel(JRadioButton.class, ids));
}
public static void main(String[] args) {
Console.run(new ButtonGroups(), 500, 300);
}
} ///:~The title for the border is taken from the name of the class, stripping off all the path information. The AbstractButton is initialized to a JButton that has the label “Failed” so if you ignore the exception message, you’ll still see the problem on screen. The getConstructor( ) method produces a Constructor object that takes the array of arguments of the types in the Class array passed to getConstructor( ). Then all you do is call newInstance( ), passing it an array of Object containing your actual arguments—in this case, just the String from the ids array. Comment
This adds a little complexity to what is a simple process. To get “exclusive or” behavior with buttons, you create a button group and add each button for which you want that behavior to the group. When you run the program, you’ll see that all the buttons except JButton exhibit this “exclusive or” behavior. Comment
You can use an Icon inside a JLabel or anything that inherits from AbstractButton (including JButton, JCheckBox, JRadioButton, and the different kinds of JMenuItem). Using Icons with JLabels is quite straightforward (you’ll see an example later). The following example explores all the additional ways you can use Icons with buttons and their descendants. Comment
You can use any gif files you want, but the ones used in this example are part of this book’s code distribution, available at www.BruceEckel.com. To open a file and bring in the image, simply create an ImageIcon and hand it the file name. From then on, you can use the resulting Icon in your program. Comment
Note that path information is hard-coded into this example; you will need to change the path to correspond to the location of the image files.
//: c13:Faces.java
// Icon behavior in Jbuttons.
// <applet code=Faces
// width=250 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class Faces extends JApplet {
// The following path information is necessary
// to run via an applet directly from the disk:
static String path =
"C:/aaa-TIJ2-distribution/code/c13/";
static Icon[] faces = {
new ImageIcon(path + "face0.gif"),
new ImageIcon(path + "face1.gif"),
new ImageIcon(path + "face2.gif"),
new ImageIcon(path + "face3.gif"),
new ImageIcon(path + "face4.gif"),
};
JButton
jb = new JButton("JButton", faces[3]),
jb2 = new JButton("Disable");
boolean mad = false;
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
jb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
if(mad) {
jb.setIcon(faces[3]);
mad = false;
} else {
jb.setIcon(faces[0]);
mad = true;
}
jb.setVerticalAlignment(JButton.TOP);
jb.setHorizontalAlignment(JButton.LEFT);
}
});
jb.setRolloverEnabled(true);
jb.setRolloverIcon(faces[1]);
jb.setPressedIcon(faces[2]);
jb.setDisabledIcon(faces[4]);
jb.setToolTipText("Yow!");
cp.add(jb);
jb2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
if(jb.isEnabled()) {
jb.setEnabled(false);
jb2.setText("Enable");
} else {
jb.setEnabled(true);
jb2.setText("Disable");
}
}
});
cp.add(jb2);
}
public static void main(String[] args) {
Console.run(new Faces(), 400, 200);
}
} ///:~An Icon can be used in many constructors, but you can also use setIcon( ) to add or change an Icon. This example also shows how a JButton (or any AbstractButton) can set the various different sorts of icons that appear when things happen to that button: when it’s pressed, disabled, or “rolled over” (the mouse moves over it without clicking). You’ll see that this gives the button a nice animated feel. Comment
The previous example added a “tool tip” to the button. Almost all of the classes that you’ll be using to create your user interfaces are derived from JComponent, which contains a method called setToolTipText(String). So, for virtually anything you place on your form, all you need to do is say (for an object jc of any JComponent-derived class):
jc.setToolTipText("My tip");and when the mouse stays over that JComponent for a predetermined period of time, a tiny box containing your text will pop up next to the mouse. Comment
This example shows the extra behavior that JTextFields are capable of:
//: c13:TextFields.java
// Text fields and Java events.
// <applet code=TextFields width=375
// height=125></applet>
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class TextFields extends JApplet {
JButton
b1 = new JButton("Get Text"),
b2 = new JButton("Set Text");
JTextField
t1 = new JTextField(30),
t2 = new JTextField(30),
t3 = new JTextField(30);
String s = new String();
UpperCaseDocument
ucd = new UpperCaseDocument();
public void init() {
t1.setDocument(ucd);
ucd.addDocumentListener(new T1());
b1.addActionListener(new B1());
b2.addActionListener(new B2());
DocumentListener dl = new T1();
t1.addActionListener(new T1A());
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b1);
cp.add(b2);
cp.add(t1);
cp.add(t2);
cp.add(t3);
}
class T1 implements DocumentListener {
public void changedUpdate(DocumentEvent e){}
public void insertUpdate(DocumentEvent e){
t2.setText(t1.getText());
t3.setText("Text: "+ t1.getText());
}
public void removeUpdate(DocumentEvent e){
t2.setText(t1.getText());
}
}
class T1A implements ActionListener {
private int count = 0;
public void actionPerformed(ActionEvent e) {
t3.setText("t1 Action Event " + count++);
}
}
class B1 implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(t1.getSelectedText() == null)
s = t1.getText();
else
s = t1.getSelectedText();
t1.setEditable(true);
}
}
class B2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
ucd.setUpperCase(false);
t1.setText("Inserted by Button 2: " + s);
ucd.setUpperCase(true);
t1.setEditable(false);
}
}
public static void main(String[] args) {
Console.run(new TextFields(), 375, 125);
}
}
class UpperCaseDocument extends PlainDocument {
boolean upperCase = true;
public void setUpperCase(boolean flag) {
upperCase = flag;
}
public void insertString(int offset,
String string, AttributeSet attributeSet)
throws BadLocationException {
if(upperCase)
string = string.toUpperCase();
super.insertString(offset,
string, attributeSet);
}
} ///:~The JTextField t3 is included as a place to report when the action listener for the JTextField t1 is fired. You’ll see that the action listener for a JTextField is fired only when you press the “enter” key. Comment
The JTextField t1 has several listeners attached to it. The T1 listener is a DocumentListener that responds to any change in the “document” (the contents of the JTextField, in this case). It automatically copies all text from t1 into t2. In addition, t1’s document is set to a derived class of PlainDocument, called UpperCaseDocument, which forces all characters to uppercase. It automatically detects backspaces and performs the deletion, adjusting the caret and handling everything as you would expect. Comment
JComponent contains a method called setBorder( ), which allows you to place various interesting borders on any visible component. The following example demonstrates a number of the different borders that are available, using a method called showBorder( ) that creates a JPanel and puts on the border in each case. Also, it uses RTTI to find the name of the border that you’re using (stripping off all the path information), then puts that name in a JLabel in the middle of the panel:
//: c13:Borders.java
// Different Swing borders.
// <applet code=Borders
// width=500 height=300></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;
public class Borders extends JApplet {
static JPanel showBorder(Border b) {
JPanel jp = new JPanel();
jp.setLayout(new BorderLayout());
String nm = b.getClass().toString();
nm = nm.substring(nm.lastIndexOf('.') + 1);
jp.add(new JLabel(nm, JLabel.CENTER),
BorderLayout.CENTER);
jp.setBorder(b);
return jp;
}
public void init() {
Container cp = getContentPane();
cp.setLayout(new GridLayout(2,4));
cp.add(showBorder(new TitledBorder("Title")));
cp.add(showBorder(new EtchedBorder()));
cp.add(showBorder(new LineBorder(Color.blue)));
cp.add(showBorder(
new MatteBorder(5,5,30,30,Color.green)));
cp.add(showBorder(
new BevelBorder(BevelBorder.RAISED)));
cp.add(showBorder(
new SoftBevelBorder(BevelBorder.LOWERED)));
cp.add(showBorder(new CompoundBorder(
new EtchedBorder(),
new LineBorder(Color.red))));
}
public static void main(String[] args) {
Console.run(new Borders(), 500, 300);
}
} ///:~You can also create your own borders and put them inside buttons, labels, etc.—anything derived from JComponent. Comment
Most of the time you’ll just want to let a JScrollPane do it’s job, but you can also control which scroll bars are allowed—vertical, horizontal, both, or neither:
//: c13:JScrollPanes.java
// Controlling the scrollbars in a JScrollPane.
// <applet code=JScrollPanes width=300 height=725>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;
public class JScrollPanes extends JApplet {
JButton
b1 = new JButton("Text Area 1"),
b2 = new JButton("Text Area 2"),
b3 = new JButton("Replace Text"),
b4 = new JButton("Insert Text");
JTextArea
t1 = new JTextArea("t1", 1, 20),
t2 = new JTextArea("t2", 4, 20),
t3 = new JTextArea("t3", 1, 20),
t4 = new JTextArea("t4", 10, 10),
t5 = new JTextArea("t5", 4, 20),
t6 = new JTextArea("t6", 10, 10);
JScrollPane
sp3 = new JScrollPane(t3,
JScrollPane.VERTICAL_SCROLLBAR_NEVER,
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER),
sp4 = new JScrollPane(t4,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER),
sp5 = new JScrollPane(t5,
JScrollPane.VERTICAL_SCROLLBAR_NEVER,
JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS),
sp6 = new JScrollPane(t6,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
class B1L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t5.append(t1.getText() + "\n");
}
}
class B2L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t2.setText("Inserted by Button 2");
t2.append(": " + t1.getText());
t5.append(t2.getText() + "\n");
}
}
class B3L implements ActionListener {
public void actionPerformed(ActionEvent e) {
String s = " Replacement ";
t2.replaceRange(s, 3, 3 + s.length());
}
}
class B4L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t2.insert(" Inserted ", 10);
}
}
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
// Create Borders for components:
Border brd = BorderFactory.createMatteBorder(
1, 1, 1, 1, Color.black);
t1.setBorder(brd);
t2.setBorder(brd);
sp3.setBorder(brd);
sp4.setBorder(brd);
sp5.setBorder(brd);
sp6.setBorder(brd);
// Initialize listeners and add components:
b1.addActionListener(new B1L());
cp.add(b1);
cp.add(t1);
b2.addActionListener(new B2L());
cp.add(b2);
cp.add(t2);
b3.addActionListener(new B3L());
cp.add(b3);
b4.addActionListener(new B4L());
cp.add(b4);
cp.add(sp3);
cp.add(sp4);
cp.add(sp5);
cp.add(sp6);
}
public static void main(String[] args) {
Console.run(new JScrollPanes(), 300, 725);
}
} ///:~Using different arguments in the JScrollPane constructor controls the scrollbars that are available. This example also dresses things up a bit using borders. Comment
The JTextPane control provides a great deal of support for editing, without much effort. The following example makes very simple use of this, ignoring the bulk of the functionality of the class:
//: c13:TextPane.java
// The JTextPane control is a little editor.
// <applet code=TextPane width=475 height=425>
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
import com.bruceeckel.util.*;
public class TextPane extends JApplet {
JButton b = new JButton("Add Text");
JTextPane tp = new JTextPane();
static Generator sg =
new Arrays2.RandStringGenerator(7);
public void init() {
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
for(int i = 1; i < 10; i++)
tp.setText(tp.getText() +
sg.next() + "\n");
}
});
Container cp = getContentPane();
cp.add(new JScrollPane(tp));
cp.add(BorderLayout.SOUTH, b);
}
public static void main(String[] args) {
Console.run(new TextPane(), 475, 425);
}
} ///:~The button just adds randomly generated text. The intent of the JTextPane is to allow text to be edited in place, so you will see that there is no append( ) method. In this case (admittedly, a poor use of the capabilities of JTextPane), the text must be captured, modified, and placed back into the pane using setText( ). Comment
As mentioned before, the default layout behavior of an applet is to use the BorderLayout. If you add something to the pane without specifying any details, it just fills the center of the pane out to the edges. However, if you specify one of the surrounding regions (NORTH, SOUTH, EAST, or WEST) as is done here, the component will fit itself into that region—in this case, the button will nest down at the bottom of the screen. Comment
Notice the built-in features of JTextPane, such as automatic line wrapping. There are lots of other features that you can look up using the JDK documentation. Comment
A check box provides a way to make a single on/off choice; it consists of a tiny box and a label. The box typically holds a little “x” (or some other indication that it is set) or is empty, depending on whether that item was selected. Comment
You’ll normally create a JCheckBox using a constructor that takes the label as an argument. You can get and set the state, and also get and set the label if you want to read or change it after the JCheckBox has been created. Comment
Whenever a JCheckBox is set or cleared, an event occurs, which you can capture the same way you do a button, by using an ActionListener. The following example uses a JTextArea to enumerate all the check boxes that have been checked:
//: c13:CheckBoxes.java
// Using JCheckBoxes.
// <applet code=CheckBoxes width=200 height=200>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class CheckBoxes extends JApplet {
JTextArea t = new JTextArea(6, 15);
JCheckBox
cb1 = new JCheckBox("Check Box 1"),
cb2 = new JCheckBox("Check Box 2"),
cb3 = new JCheckBox("Check Box 3");
public void init() {
cb1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
trace("1", cb1);
}
});
cb2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
trace("2", cb2);
}
});
cb3.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
trace("3", cb3);
}
});
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(new JScrollPane(t));
cp.add(cb1);
cp.add(cb2);
cp.add(cb3);
}
void trace(String b, JCheckBox cb) {
if(cb.isSelected())
t.append("Box " + b + " Set\n");
else
t.append("Box " + b + " Cleared\n");
}
public static void main(String[] args) {
Console.run(new CheckBoxes(), 200, 200);
}
} ///:~The trace( ) method sends the name of the selected JCheckBox and its current state to the JTextArea using append( ), so you’ll see a cumulative list of the checkboxes that were selected and what their state is. Comment
The concept of a radio button in GUI programming comes from pre-electronic car radios with mechanical buttons: when you push one in, any other button that was pressed pops out. Thus, it allows you to force a single choice among many. Comment
All you need to do to set up an associated group of JRadioButtons is to add them to a ButtonGroup (you can have any number of ButtonGroups on a form). One of the buttons can optionally have its starting state set to true (using the second argument in the constructor). If you try to set more than one radio button to true then only the final one set will be true. Comment
Here’s a simple example of the use of radio buttons. Note that you capture radio button events like all others:
//: c13:RadioButtons.java
// Using JRadioButtons.
// <applet code=RadioButtons
// width=200 height=100> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class RadioButtons extends JApplet {
JTextField t = new JTextField(15);
ButtonGroup g = new ButtonGroup();
JRadioButton
rb1 = new JRadioButton("one", false),
rb2 = new JRadioButton("two", false),
rb3 = new JRadioButton("three", false);
ActionListener al = new ActionListener() {
public void actionPerformed(ActionEvent e) {
t.setText("Radio button " +
((JRadioButton)e.getSource()).getText());
}
};
public void init() {
rb1.addActionListener(al);
rb2.addActionListener(al);
rb3.addActionListener(al);
g.add(rb1); g.add(rb2); g.add(rb3);
t.setEditable(false);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
cp.add(rb1);
cp.add(rb2);
cp.add(rb3);
}
public static void main(String[] args) {
Console.run(new RadioButtons(), 200, 100);
}
} ///:~To display the state, a text field is used. This field is set to noneditable because it’s used only to display data, not to collect it. Thus it is an alternative to using a JLabel. Comment
Like a group of radio buttons, a drop-down list is a way to force the user to select only one element from a group of possibilities. However, it’s a more compact way to accomplish this, and it’s easier to change the elements of the list without surprising the user. (You can change radio buttons dynamically, but that tends to be visibly jarring). Comment
Java’s JComboBox box is not like the combo box in Windows, which lets you select from a list or type in your own selection. With a JComboBox box you choose one and only one element from the list. In the following example, the JComboBox box starts with a certain number of entries and then new entries are added to the box when a button is pressed. Comment
//: c13:ComboBoxes.java
// Using drop-down lists.
// <applet code=ComboBoxes
// width=200 height=100> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class ComboBoxes extends JApplet {
String[] description = { "Ebullient", "Obtuse",
"Recalcitrant", "Brilliant", "Somnescent",
"Timorous", "Florid", "Putrescent" };
JTextField t = new JTextField(15);
JComboBox c = new JComboBox();
JButton b = new JButton("Add items");
int count = 0;
public void init() {
for(int i = 0; i < 4; i++)
c.addItem(description[count++]);
t.setEditable(false);
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
if(count < description.length)
c.addItem(description[count++]);
}
});
c.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
t.setText("index: "+ c.getSelectedIndex()
+ " " + ((JComboBox)e.getSource())
.getSelectedItem());
}
});
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
cp.add(c);
cp.add(b);
}
public static void main(String[] args) {
Console.run(new ComboBoxes(), 200, 100);
}
} ///:~The JTextField displays the “selected index,” which is the sequence number of the currently selected element, as well as the label on the radio button. Comment
List boxes are significantly different from JComboBox boxes, and not just in appearance. While a JComboBox box drops down when you activate it, a JList occupies some fixed number of lines on a screen all the time and doesn’t change. If you want to see the items in a list, you simply call getSelectedValues( ), which produces an array of String of the items that have been selected. Comment
A JList allows multiple selection: if you control-click on more than one item (holding down the “control” key while performing additional mouse clicks) the original item stays highlighted and you can select as many as you want. If you select an item, then shift-click on another item, all the items in the span between the two are selected. To remove an item from a group you can control-click it. Comment
//: c13:List.java
// <applet code=List width=250
// height=375> </applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;
public class List extends JApplet {
String[] flavors = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
DefaultListModel lItems=new DefaultListModel();
JList lst = new JList(lItems);
JTextArea t = new JTextArea(flavors.length,20);
JButton b = new JButton("Add Item");
ActionListener bl = new ActionListener() {
public void actionPerformed(ActionEvent e) {
if(count < flavors.length) {
lItems.add(0, flavors[count++]);
} else {
// Disable, since there are no more
// flavors left to be added to the List
b.setEnabled(false);
}
}
};
ListSelectionListener ll =
new ListSelectionListener() {
public void valueChanged(
ListSelectionEvent e) {
t.setText("");
Object[] items=lst.getSelectedValues();
for(int i = 0; i < items.length; i++)
t.append(items[i] + "\n");
}
};
int count = 0;
public void init() {
Container cp = getContentPane();
t.setEditable(false);
cp.setLayout(new FlowLayout());
// Create Borders for components:
Border brd = BorderFactory.createMatteBorder(
1, 1, 2, 2, Color.black);
lst.setBorder(brd);
t.setBorder(brd);
// Add the first four items to the List
for(int i = 0; i < 4; i++)
lItems.addElement(flavors[count++]);
// Add items to the Content Pane for Display
cp.add(t);
cp.add(lst);
cp.add(b);
// Register event listeners
lst.addListSelectionListener(ll);
b.addActionListener(bl);
}
public static void main(String[] args) {
Console.run(new List(), 250, 375);
}
} ///:~When you press the button it adds items to the top of the list (because addItem( )’s second argument is 0). Comment
You can see that borders have also been added to the lists. Comment
If you just want to put an array of Strings into a JList, there’s a much simpler solution: you pass the array to the JList constructor, and it builds the list automatically. The only reason for using the “list model” in the above example is so that the list could be manipulated during the execution of the program. Comment
JLists do not automatically provide direct support for scrolling. Of course, all you need to do is wrap the JList in a JScrollPane and all the details are automatically managed for you. Comment
The JTabbedPane allows you to create a “tabbed dialog,” which has file-folder tabs running across one edge, and all you have to do is press a tab to bring forward a different dialog.
//: c13:TabbedPane1.java
// Demonstrates the Tabbed Pane.
// <applet code=TabbedPane1
// width=350 height=200> </applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class TabbedPane1 extends JApplet {
String[] flavors = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
JTabbedPane tabs = new JTabbedPane();
JTextField txt = new JTextField(20);
public void init() {
for(int i = 0; i < flavors.length; i++)
tabs.addTab(flavors[i],
new JButton("Tabbed pane " + i));
tabs.addChangeListener(new ChangeListener(){
public void stateChanged(ChangeEvent e) {
txt.setText("Tab selected: " +
tabs.getSelectedIndex());
}
});
Container cp = getContentPane();
cp.add(BorderLayout.SOUTH, txt);
cp.add(tabs);
}
public static void main(String[] args) {
Console.run(new TabbedPane1(), 350, 200);
}
} ///:~In Java, the use of some sort of “tabbed panel” mechanism is quite important because in applet programming the use of pop-up dialogs is discouraged by automatically adding a little warning to any dialog that pops up out of an applet. Comment
When you run the program you’ll see that the JTabbedPane automatically stacks the tabs if there are too many of them to fit on one row. You can see this by resizing the window when you run the program from the console command line. Comment
Windowing environments commonly contain a standard set of message boxes that allow you to quickly post information to the user or to capture information from the user. In Swing, these message boxes are contained in JOptionPane. You have many different possibilities (some quite sophisticated), but the ones you’ll most commonly use are probably the message dialog and confirmation dialog, invoked using the static JOptionPane.showMessageDialog( ) and JOptionPane. showConfirmDialog( ). The following example shows a subset of the message boxes available with JOptionPane:
//: c13:MessageBoxes.java
// Demonstrates JoptionPane.
// <applet code=MessageBoxes
// width=200 height=150> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class MessageBoxes extends JApplet {
JButton[] b = { new JButton("Alert"),
new JButton("Yes/No"), new JButton("Color"),
new JButton("Input"), new JButton("3 Vals")
};
JTextField txt = new JTextField(15);
ActionListener al = new ActionListener() {
public void actionPerformed(ActionEvent e){
String id =
((JButton)e.getSource()).getText();
if(id.equals("Alert"))
JOptionPane.showMessageDialog(null,
"There's a bug on you!", "Hey!",
JOptionPane.ERROR_MESSAGE);
else if(id.equals("Yes/No"))
JOptionPane.showConfirmDialog(null,
"or no", "choose yes",
JOptionPane.YES_NO_OPTION);
else if(id.equals("Color")) {
Object[] options = { "Red", "Green" };
int sel = JOptionPane.showOptionDialog(
null, "Choose a Color!", "Warning",
JOptionPane.DEFAULT_OPTION,
JOptionPane.WARNING_MESSAGE, null,
options, options[0]);
if(sel != JOptionPane.CLOSED_OPTION)
txt.setText(
"Color Selected: " + options[sel]);
} else if(id.equals("Input")) {
String val = JOptionPane.showInputDialog(
"How many fingers do you see?");
txt.setText(val);
} else if(id.equals("3 Vals")) {
Object[] selections = {
"First", "Second", "Third" };
Object val = JOptionPane.showInputDialog(
null, "Choose one", "Input",
JOptionPane.INFORMATION_MESSAGE,
null, selections, selections[0]);
if(val != null)
txt.setText(
val.toString());
}
}
};
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
for(int i = 0; i < b.length; i++) {
b[i].addActionListener(al);
cp.add(b[i]);
}
cp.add(txt);
}
public static void main(String[] args) {
Console.run(new MessageBoxes(), 200, 200);
}
} ///:~To be able to write a single ActionListener, I’ve used the somewhat risky approach of checking the String labels on the buttons. The problem with this is that it’s easy to get the label a little bit wrong, typically in capitalization, and this bug can be hard to spot. Comment
Note that showOptionDialog( ) and showInputDialog( ) provide return objects that contain the value entered by the user. Comment
Each component capable of holding a menu, including JApplet, JFrame, JDialog, and their descendants, has a setJMenuBar( ) method that accepts a JMenuBar (you can have only one JMenuBar on a particular component). You add JMenus to the JMenuBar, and JMenuItems to the JMenus. Each JMenuItem can have an ActionListener attached to it, to be fired when that menu item is selected. Comment
Unlike a system that uses resources, with Java and Swing you must hand assemble all the menus in source code. Here is a very simple menu example:
//: c13:SimpleMenus.java
// <applet code=SimpleMenus
// width=200 height=75> </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class SimpleMenus extends JApplet {
JTextField t = new JTextField(15);
ActionListener al = new ActionListener() {
public void actionPerformed(ActionEvent e){
t.setText(
((JMenuItem)e.getSource()).getText());
}
};
JMenu[] menus = { new JMenu("Winken"),
new JMenu("Blinken"), new JMenu("Nod") };
JMenuItem[] items = {
new JMenuItem("Fee"), new JMenuItem("Fi"),
new JMenuItem("Fo"), new JMenuItem("Zip"),
new JMenuItem("Zap"), new JMenuItem("Zot"),
new JMenuItem("Olly"), new JMenuItem("Oxen"),
new JMenuItem("Free") };
public void init() {
for(int i = 0; i < items.length; i++) {
items[i].addActionListener(al);
menus[i%3].add(items[i]);
}
JMenuBar mb = new JMenuBar();
for(int i = 0; i < menus.length; i++)
mb.add(menus[i]);
setJMenuBar(mb);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
}
public static void main(String[] args) {
Console.run(new SimpleMenus(), 200, 75);
}
} ///:~The use of the modulus operator in “i%3” distributes the menu items among the three JMenus. Each JMenuItem must have an ActionListener attached to it; here, the same ActionListener is used everywhere but you’ll usually need an individual one for each JMenuItem. Comment
JMenuItem inherits AbstractButton, so it has some buttonlike behaviors. By itself, it provides an item that can be placed on a drop-down menu. There are also three types inherited from JMenuItem: JMenu to hold other JMenuItems (so you can have cascading menus), JCheckBoxMenuItem, which produces a checkmark to indicate whether that menu item is selected, and JRadioButtonMenuItem, which contains a radio button. Comment
As a more sophisticated example, here are the ice cream flavors again, used to create menus. This example also shows cascading menus, keyboard mnemonics, JCheckBoxMenuItems, and the way you can dynamically change menus:
//: c13:Menus.java
// Submenus, checkbox menu items, swapping menus,
// mnemonics (shortcuts) and action commands.
// <applet code=Menus width=300
// height=100> </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class Menus extends JApplet {
String[] flavors = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
JTextField t = new JTextField("No flavor", 30);
JMenuBar mb1 = new JMenuBar();
JMenu
f = new JMenu("File"),
m = new JMenu("Flavors"),
s = new JMenu("Safety");
// Alternative approach:
JCheckBoxMenuItem[] safety = {
new JCheckBoxMenuItem("Guard"),
new JCheckBoxMenuItem("Hide")
};
JMenuItem[] file = {
new JMenuItem("Open"),
};
// A second menu bar to swap to:
JMenuBar mb2 = new JMenuBar();
JMenu fooBar = new JMenu("fooBar");
JMenuItem[] other = {
// Adding a menu shortcut (mnemonic) is very
// simple, but only JMenuItems can have them
// in their constructors:
new JMenuItem("Foo", KeyEvent.VK_F),
new JMenuItem("Bar", KeyEvent.VK_A),
// No shortcut:
new JMenuItem("Baz"),
};
JButton b = new JButton("Swap Menus");
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
JMenuBar m = getJMenuBar();
setJMenuBar(m == mb1 ? mb2 : mb1);
validate(); // Refresh the frame
}
}
class ML implements ActionListener {
public void actionPerformed(ActionEvent e) {
JMenuItem target = (JMenuItem)e.getSource();
String actionCommand =
target.getActionCommand();
if(actionCommand.equals("Open")) {
String s = t.getText();
boolean chosen = false;
for(int i = 0; i < flavors.length; i++)
if(s.equals(flavors[i])) chosen = true;
if(!chosen)
t.setText("Choose a flavor first!");
else
t.setText("Opening "+ s +". Mmm, mm!");
}
}
}
class FL implements ActionListener {
public void actionPerformed(ActionEvent e) {
JMenuItem target = (JMenuItem)e.getSource();
t.setText(target.getText());
}
}
// Alternatively, you can create a different
// class for each different MenuItem. Then you
// Don't have to figure out which one it is:
class FooL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Foo selected");
}
}
class BarL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Bar selected");
}
}
class BazL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Baz selected");
}
}
class CMIL implements ItemListener {
public void itemStateChanged(ItemEvent e) {
JCheckBoxMenuItem target =
(JCheckBoxMenuItem)e.getSource();
String actionCommand =
target.getActionCommand();
if(actionCommand.equals("Guard"))
t.setText("Guard the Ice Cream! " +
"Guarding is " + target.getState());
else if(actionCommand.equals("Hide"))
t.setText("Hide the Ice Cream! " +
"Is it cold? " + target.getState());
}
}
public void init() {
ML ml = new ML();
CMIL cmil = new CMIL();
safety[0].setActionCommand("Guard");
safety[0].setMnemonic(KeyEvent.VK_G);
safety[0].addItemListener(cmil);
safety[1].setActionCommand("Hide");
safety[1].setMnemonic(KeyEvent.VK_H);
safety[1].addItemListener(cmil);
other[0].addActionListener(new FooL());
other[1].addActionListener(new BarL());
other[2].addActionListener(new BazL());
FL fl = new FL();
for(int i = 0; i < flavors.length; i++) {
JMenuItem mi = new JMenuItem(flavors[i]);
mi.addActionListener(fl);
m.add(mi);
// Add separators at intervals:
if((i+1) % 3 == 0)
m.addSeparator();
}
for(int i = 0; i < safety.length; i++)
s.add(safety[i]);
s.setMnemonic(KeyEvent.VK_A);
f.add(s);
f.setMnemonic(KeyEvent.VK_F);
for(int i = 0; i < file.length; i++) {
file[i].addActionListener(fl);
f.add(file[i]);
}
mb1.add(f);
mb1.add(m);
setJMenuBar(mb1);
t.setEditable(false);
Container cp = getContentPane();
cp.add(t, BorderLayout.CENTER);
// Set up the system for swapping menus:
b.addActionListener(new BL());
b.setMnemonic(KeyEvent.VK_S);
cp.add(b, BorderLayout.NORTH);
for(int i = 0; i < other.length; i++)
fooBar.add(other[i]);
fooBar.setMnemonic(KeyEvent.VK_B);
mb2.add(fooBar);
}
public static void main(String[] args) {
Console.run(new Menus(), 300, 100);
}
} ///:~In this program I placed the menu items into arrays and then stepped through each array calling add( ) for each JMenuItem. This makes adding or subtracting a menu item somewhat less tedious. Comment
This program creates not one but two JMenuBars to demonstrate that menu bars can be actively swapped while the program is running. You can see how a JMenuBar is made up of JMenus, and each JMenu is made up of JMenuItems, JCheckBoxMenuItems, or even other JMenus (which produce submenus). When a JMenuBar is assembled it can be installed into the current program with the setJMenuBar( ) method. Note that when the button is pressed, it checks to see which menu is currently installed by calling getJMenuBar( ), then it puts the other menu bar in its place. Comment
When testing for “Open,” notice that spelling and capitalization are critical, but Java signals no error if there is no match with “Open.” This kind of string comparison is a source of programming errors. Comment
The checking and unchecking of the menu items is taken care of automatically. The code handling the JCheckBoxMenuItems shows two different ways to determine what was checked: string matching (which, as mentioned above, isn’t a very safe approach although you’ll see it used) and matching on the event target object. As shown, the getState( ) method can be used to reveal the state. You can also change the state of a JCheckBoxMenuItem with setState( ). Comment
The events for menus are a bit inconsistent and can lead to confusion: JMenuItems use ActionListeners, but JCheckboxMenuItems use ItemListeners. The JMenu objects can also support ActionListeners, but that’s not usually helpful. In general, you’ll attach listeners to each JMenuItem, JCheckBoxMenuItem, or JRadioButtonMenuItem, but the example shows ItemListeners and ActionListeners attached to the various menu components. Comment
Swing supports mnemonics, or “keyboard shortcuts,” so you can select anything derived from AbstractButton (button, menu item, etc.) using the keyboard instead of the mouse. These are quite simple: for JMenuItem you can use the overloaded constructor that takes as a second argument the identifier for the key. However, most AbstractButtons do not have constructors like this so the more general way to solve the problem is to use the setMnemonic( ) method. The example above adds mnemonics to the button and some of the menu items; shortcut indicators automatically appear on the components. Comment
You can also see the use of setActionCommand( ). This seems a bit strange because in each case the “action command” is exactly the same as the label on the menu component. Why not just use the label instead of this alternative string? The problem is internationalization. If you retarget this program to another language, you want to change only the label in the menu, and not change the code (which would no doubt introduce new errors). So to make this easy for code that checks the text string associated with a menu component, the “action command” can be immutable while the menu label can change. All the code works with the “action command,” so it’s unaffected by changes to the menu labels. Note that in this program, not all the menu components are examined for their action commands, so those that aren’t don’t have their action command set. Comment
The bulk of the work happens in the listeners. BL performs the JMenuBar swapping. In ML, the “figure out who rang” approach is taken by getting the source of the ActionEvent and casting it to a JMenuItem, then getting the action command string to pass it through a cascaded if statement. Comment
The FL listener is simple even though it’s handling all the different flavors in the flavor menu. This approach is useful if you have enough simplicity in your logic, but in general, you’ll want to take the approach used with FooL, BarL, and BazL, in which they are each attached to only a single menu component so no extra detection logic is necessary and you know exactly who called the listener. Even with the profusion of classes generated this way, the code inside tends to be smaller and the process is more foolproof. Comment
You can see that menu code quickly gets long-winded and messy. This is another case where the use of a GUI builder is the appropriate solution. A good tool will also handle the maintenance of the menus. Comment
The most straightforward way to implement a JPopupMenu is to create an inner class that extends MouseAdapter, then add an object of that inner class to each component that you want to produce pop-up behavior:
//: c13:Popup.java
// Creating popup menus with Swing.
// <applet code=Popup
// width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class Popup extends JApplet {
JPopupMenu popup = new JPopupMenu();
JTextField t = new JTextField(10);
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
ActionListener al = new ActionListener() {
public void actionPerformed(ActionEvent e){
t.setText(
((JMenuItem)e.getSource()).getText());
}
};
JMenuItem m = new JMenuItem("Hither");
m.addActionListener(al);
popup.add(m);
m = new JMenuItem("Yon");
m.addActionListener(al);
popup.add(m);
m = new JMenuItem("Afar");
m.addActionListener(al);
popup.add(m);
popup.addSeparator();
m = new JMenuItem("Stay Here");
m.addActionListener(al);
popup.add(m);
PopupListener pl = new PopupListener();
addMouseListener(pl);
t.addMouseListener(pl);
}
class PopupListener extends MouseAdapter {
public void mousePressed(MouseEvent e) {
maybeShowPopup(e);
}
public void mouseReleased(MouseEvent e) {
maybeShowPopup(e);
}
private void maybeShowPopup(MouseEvent e) {
if(e.isPopupTrigger()) {
popup.show(
e.getComponent(), e.getX(), e.getY());
}
}
}
public static void main(String[] args) {
Console.run(new Popup(), 300, 200);
}
} ///:~The same ActionListener is added to each JMenuItem, so that it fetches the text from the menu label and inserts it into the JTextField. Comment
In a good GUI framework, drawing should be reasonably easy—and it is, in the Swing library. The problem with any drawing example is that the calculations that determine where things go are typically a lot more complicated that the calls to the drawing routines, and these calculations are often mixed together with the drawing calls so it can seem that the interface is more complicated than it actually is. Comment
For simplicity, consider the problem of representing data on the screen—here, the data will be provided by the built-in Math.sin( ) method which is a mathematical sine function. To make things a little more interesting, and to further demonstrate how easy it is to use Swing components, a slider will be placed at the bottom of the form to dynamically control the number of sine wave cycles that are displayed. In addition, if you resize the window, you’ll see that the sine wave refits itself to the new window size. Comment
Although any JComponent may be painted and thus used as a canvas, if you just want a straightforward drawing surface you will typically inherit from a JPanel. The only method you need to override is paintComponent( ), which is called whenever that component must be repainted (you normally don’t need to worry about this, as the decision is managed by Swing). When it is called, Swing passes a Graphics object to the method, and you can then use this object to draw or paint on the surface. Comment
In the following example, all the intelligence concerning painting is in the SineDraw class; the SineWave class simply configures the program and the slider control. Inside SineDraw, the setCycles( ) method provides a hook to allow another object—the slider control, in this case—to control the number of cycles.
//: c13:SineWave.java
// Drawing with Swing, using a JSlider.
// <applet code=SineWave
// width=700 height=400></applet>
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
class SineDraw extends JPanel {
static final int SCALEFACTOR = 200;
int cycles;
int points;
double[] sines;
int[] pts;
SineDraw() { setCycles(5); }
public void setCycles(int newCycles) {
cycles = newCycles;
points = SCALEFACTOR * cycles * 2;
sines = new double[points];
for(int i = 0; i < points; i++) {
double radians = (Math.PI/SCALEFACTOR) * i;
sines[i] = Math.sin(radians);
}
repaint();
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
int maxWidth = getWidth();
double hstep = (double)maxWidth/(double)points;
int maxHeight = getHeight();
pts = new int[points];
for(int i = 0; i < points; i++)
pts[i] = (int)(sines[i] * maxHeight/2 * .95
+ maxHeight/2);
g.setColor(Color.red);
for(int i = 1; i < points; i++) {
int x1 = (int)((i - 1) * hstep);
int x2 = (int)(i * hstep);
int y1 = pts[i-1];
int y2 = pts[i];
g.drawLine(x1, y1, x2, y2);
}
}
}
public class SineWave extends JApplet {
SineDraw sines = new SineDraw();
JSlider adjustCycles = new JSlider(1, 30, 5);
public void init() {
Container cp = getContentPane();
cp.add(sines);
adjustCycles.addChangeListener(
new ChangeListener(){
public void stateChanged(ChangeEvent e) {
sines.setCycles(
((JSlider)e.getSource()).getValue());
}
});
cp.add(BorderLayout.SOUTH, adjustCycles);
}
public static void main(String[] args) {
Console.run(new SineWave(), 700, 400);
}
} ///:~All of the data members and arrays are used in the calculation of the sine wave points: cycles indicates the number of complete sine waves desired, points contains the total number of points that will be graphed, sines contains the sine function values, and pts contains the y-coordinates of the points that will be drawn on the JPanel. The setCycles( ) method creates the arrays according to the number of points needed and fills the sines array with numbers. By calling repaint( ) , setCycles( ) forces paintComponent( ) to be called so the rest of the calculation and redraw will take place. Comment
The first thing you must do when you override paintComponent( ) is to call the base-class version of the method. Then you are free to do whatever you like; normally, this means using the Graphics methods that you can find in the documentation for java.awt.Graphics (in the HTML documentation from java.sun.com) to draw and paint pixels onto the JPanel. Here, you can see that almost all the code is involved in performing the calculations; the only two method calls that actually manipulate the screen are setColor( ) and drawLine( ). You will probably have a similar experience when creating your own program that displays graphical data—you’ll spend most of your time figuring out what it is you want to draw, but the actual drawing process will be quite simple. Comment
When I created this program, the bulk of my time was spent in getting the sine wave to display. Once I did that, I thought it would be nice to be able to dynamically change the number of cycles. My programming experiences when trying to do such things in other languages made me a bit reluctant to try this, but it turned out to be the easiest part of the project. I created a JSlider (the arguments are the left-most value of the JSlider, the right-most value, and the starting value, respectively, but there are other constructors as well) and dropped it into the JApplet. Then I looked at the HTML documentation and noticed that the only listener was the addChangeListener, which was triggered whenever the slider was changed enough for it to produce a different value. The only method for this was the obviously named stateChanged( ), which provided a ChangeEvent object so that I could look backward to the source of the change and find the new value. By calling the sines object’s setCycles( ), the new value was incorporated and the JPanel redrawn. Comment
In general, you will find that most of your Swing problems can be solved by following a similar process, and you’ll find that it’s generally quite simple, even if you haven’t used a particular component before. Comment
If your problem is more complex, there are other more sophisticated alternatives for drawing, including third-party JavaBeans components and the Java 2D API. These solutions are beyond the scope of this book, but you should look them up if your drawing code becomes too onerous. Comment
A dialog box is a window that pops up out of another window. Its purpose is to deal with some specific issue without cluttering the original window with those details. Dialog boxes are heavily used in windowed programming environments, but less frequently used in applets. Comment
To create a dialog box, you inherit from JDialog, which is just another kind of Window, like a JFrame. A JDialog has a layout manager (which defaults to BorderLayout) and you add event listeners to deal with events. One significant difference when windowClosing( ) is called is that you don’t want to shut down the application. Instead, you release the resources used by the dialog’s window by calling dispose( ). Here’s a very simple example:
//: c13:Dialogs.java
// Creating and using Dialog Boxes.
// <applet code=Dialogs width=125 height=75>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
class MyDialog extends JDialog {
public MyDialog(JFrame parent) {
super(parent, "My dialog", true);
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(new JLabel("Here is my dialog"));
JButton ok = new JButton("OK");
ok.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
dispose(); // Closes the dialog
}
});
cp.add(ok);
setSize(150,125);
}
}
public class Dialogs extends JApplet {
JButton b1 = new JButton("Dialog Box");
MyDialog dlg = new MyDialog(null);
public void init() {
b1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
dlg.show();
}
});
getContentPane().add(b1);
}
public static void main(String[] args) {
Console.run(new Dialogs(), 125, 75);
}
} ///:~Once the JDialog is created, the show( ) method must be called to display and activate it. For the dialog to close, it must call dispose( ). Comment
You’ll see that anything that pops up out of an applet, including dialog boxes, is “untrusted.” That is, you get a warning in the window that’s been popped up. This is because, in theory, it would be possible to fool the user into thinking that they’re dealing with a regular native application and to get them to type in their credit card number, which then goes across the Web. An applet is always attached to a Web page and visible within your Web browser, while a dialog box is detached—so in theory, it could be possible. As a result it is not so common to see an applet that uses a dialog box. Comment
The following example is more complex; the dialog box is made up of a grid (using GridLayout) of a special kind of button that is defined here as class ToeButton. This button draws a frame around itself and, depending on its state, a blank, an “x,” or an “o” in the middle. It starts out blank, and then depending on whose turn it is, changes to an “x” or an “o.” However, it will also flip back and forth between “x” and “o” when you click on the button. (This makes the tic-tac-toe concept only slightly more annoying than it already is.) In addition, the dialog box can be set up for any number of rows and columns by changing numbers in the main application window.
//: c13:TicTacToe.java
// Demonstration of dialog boxes
// and creating your own components.
// <applet code=TicTacToe
// width=200 height=100></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class TicTacToe extends JApplet {
JTextField
rows = new JTextField("3"),
cols = new JTextField("3");
static final int BLANK = 0, XX = 1, OO = 2;
class ToeDialog extends JDialog {
int turn = XX; // Start with x's turn
// w = number of cells wide
// h = number of cells high
public ToeDialog(int w, int h) {
setTitle("The game itself");
Container cp = getContentPane();
cp.setLayout(new GridLayout(w, h));
for(int i = 0; i < w * h; i++)
cp.add(new ToeButton());
setSize(w * 50, h * 50);
// JDK 1.3 close dialog:
//#setDefaultCloseOperation(
//# DISPOSE_ON_CLOSE);
// JDK 1.2 close dialog:
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e){
dispose();
}
});
}
class ToeButton extends JPanel {
int state = BLANK;
public ToeButton() {
addMouseListener(new ML());
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
int x1 = 0;
int y1 = 0;
int x2 = getSize().width - 1;
int y2 = getSize().height - 1;
g.drawRect(x1, y1, x2, y2);
x1 = x2/4;
y1 = y2/4;
int wide = x2/2;
int high = y2/2;
if(state == XX) {
g.drawLine(x1, y1,
x1 + wide, y1 + high);
g.drawLine(x1, y1 + high,
x1 + wide, y1);
}
if(state == OO) {
g.drawOval(x1, y1,
x1 + wide/2, y1 + high/2);
}
}
class ML extends MouseAdapter {
public void mousePressed(MouseEvent e) {
if(state == BLANK) {
state = turn;
turn = (turn == XX ? OO : XX);
}
else
state = (state == XX ? OO : XX);
repaint();
}
}
}
}
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
JDialog d = new ToeDialog(
Integer.parseInt(rows.getText()),
Integer.parseInt(cols.getText()));
d.setVisible(true);
}
}
public void init() {
JPanel p = new JPanel();
p.setLayout(new GridLayout(2,2));
p.add(new JLabel("Rows", JLabel.CENTER));
p.add(rows);
p.add(new JLabel("Columns", JLabel.CENTER));
p.add(cols);
Container cp = getContentPane();
cp.add(p, BorderLayout.NORTH);
JButton b = new JButton("go");
b.addActionListener(new BL());
cp.add(b, BorderLayout.SOUTH);
}
public static void main(String[] args) {
Console.run(new TicTacToe(), 200, 100);
}
} ///:~Because statics can only be at the outer level of the class, inner classes cannot have static data or nested classes. Comment
The paintComponent( ) method draws the square around the panel, and the “x” or the “o.” This is full of tedious calculations, but it’s straightforward. Comment
A mouse click is captured by the MouseListener, which first checks to see if the panel has anything written on it. If not, the parent window is queried to find out whose turn it is and that is used to establish the state of the ToeButton. Via the inner class mechanism, the ToeButton then reaches back into the parent and changes the turn. If the button is already displaying an “x” or an “o” then that is flopped. You can see in these calculations the convenient use of the ternary if-else described in Chapter 3. After a state change, the ToeButton is repainted. Comment
The constructor for ToeDialog is quite simple: it adds into a GridLayout as many buttons as you request, then resizes it for 50 pixels on a side for each button. Comment
TicTacToe sets up the whole application by creating the JTextFields (for inputting the rows and columns of the button grid) and the “go” button with its ActionListener. When the button is pressed, the data in the JTextFields must be fetched, and, since they are in String form, turned into ints using the static Integer.parseInt( ) method. Comment
Some operating systems have a number of special built-in dialog boxes to handle the selection of things such as fonts, colors, printers, and the like. Virtually all graphical operating systems support the opening and saving of files, however, and so Java’s JFileChooser encapsulates these for easy use. Comment
The following application exercises two forms of JFileChooser dialogs, one for opening and one for saving. Most of the code should by now be familiar, and all the interesting activities happen in the action listeners for the two different button clicks:
//: c13:FileChooserTest.java
// Demonstration of File dialog boxes.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class FileChooserTest extends JFrame {
JTextField
filename = new JTextField(),
dir = new JTextField();
JButton
open = new JButton("Open"),
save = new JButton("Save");
public FileChooserTest() {
JPanel p = new JPanel();
open.addActionListener(new OpenL());
p.add(open);
save.addActionListener(new SaveL());
p.add(save);
Container cp = getContentPane();
cp.add(p, BorderLayout.SOUTH);
dir.setEditable(false);
filename.setEditable(false);
p = new JPanel();
p.setLayout(new GridLayout(2,1));
p.add(filename);
p.add(dir);
cp.add(p, BorderLayout.NORTH);
}
class OpenL implements ActionListener {
public void actionPerformed(ActionEvent e) {
JFileChooser c = new JFileChooser();
// Demonstrate "Open" dialog:
int rVal =
c.showOpenDialog(FileChooserTest.this);
if(rVal == JFileChooser.APPROVE_OPTION) {
filename.setText(
c.getSelectedFile().getName());
dir.setText(
c.getCurrentDirectory().toString());
}
if(rVal == JFileChooser.CANCEL_OPTION) {
filename.setText("You pressed cancel");
dir.setText("");
}
}
}
class SaveL implements ActionListener {
public void actionPerformed(ActionEvent e) {
JFileChooser c = new JFileChooser();
// Demonstrate "Save" dialog:
int rVal =
c.showSaveDialog(FileChooserTest.this);
if(rVal == JFileChooser.APPROVE_OPTION) {
filename.setText(
c.getSelectedFile().getName());
dir.setText(
c.getCurrentDirectory().toString());
}
if(rVal == JFileChooser.CANCEL_OPTION) {
filename.setText("You pressed cancel");
dir.setText("");
}
}
}
public static void main(String[] args) {
Console.run(new FileChooserTest(), 250, 110);
}
} ///:~Note that there are many variations you can apply to JFileChooser, including filters to narrow the file names that you will allow. Comment
For an “open file” dialog, you call showOpenDialog( ), and for a “save file” dialog you call showSaveDialog( ). These commands don’t return until the dialog is closed. The JFileChooser object still exists, so you can read data from it. The methods getSelectedFile( ) and getCurrentDirectory( ) are two ways you can interrogate the results of the operation. If these return null it means the user canceled out of the dialog. Comment
Any component that can take text can also take HTML text, which it will reformat according to HTML rules. This means you can very easily add fancy text to a Swing component. For example,
//: c13:HTMLButton.java
// Putting HTML text on Swing components.
// <applet code=HTMLButton width=200 height=500>
// </applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class HTMLButton extends JApplet {
JButton b = new JButton("<html><b><font size=+2>" +
"<center>Hello!<br><i>Press me now!");
public void init() {
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
getContentPane().add(new JLabel("<html>"+
"<i><font size=+4>Kapow!"));
// Force a re-layout to
// include the new label:
validate();
}
});
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(b);
}
public static void main(String[] args) {
Console.run(new HTMLButton(), 200, 500);
}
} ///:~You must start the text with “<html>,” and then you can use normal HTML tags. Note that you are not forced to include the normal closing tags. Comment
The ActionListener adds a new JLabel to the form, which also contains HTML text. However, this label is not added during init( ) so you must call the container’s validate( ) method in order to force a re-layout of the components (and thus the display of the new label). Comment
You can also use HTML text for JTabbedPane, JMenuItem, JToolTip, JRadioButton and JCheckBox. Comment
A slider (which has already been used in the sine wave example) allows the user to input data by moving a point back and forth, which is intuitive in some situations (volume controls, for example). A progress bar displays data in a relative fashion from “full” to “empty” so the user gets a perspective. My favorite example for these is to simply hook the slider to the progress bar so when you move the slider the progress bar changes accordingly:
//: c13:Progress.java
// Using progress bars and sliders.
// <applet code=Progress
// width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import javax.swing.border.*;
import com.bruceeckel.swing.*;
public class Progress extends JApplet {
JProgressBar pb = new JProgressBar();
JSlider sb =
new JSlider(JSlider.HORIZONTAL, 0, 100, 60);
public void init() {
Container cp = getContentPane();
cp.setLayout(new GridLayout(2,1));
cp.add(pb);
sb.setValue(0);
sb.setPaintTicks(true);
sb.setMajorTickSpacing(20);
sb.setMinorTickSpacing(5);
sb.setBorder(new TitledBorder("Slide Me"));
pb.setModel(sb.getModel()); // Share model
cp.add(sb);
}
public static void main(String[] args) {
Console.run(new Progress(), 300, 200);
}
} ///:~The key to hooking the two components together is in sharing their model, in the line: Comment
pb.setModel(sb.getModel());
Of course, you could also control the two using a listener, but this is more straightforward for simple situations.
The JProgressBar is fairly straightforward, but the JSlider has a lot of options, such as the orientation and major and minor tick marks. Notice how straightforward it is to add a titled border. Comment
Using a JTree can be as simple as saying:
add(new JTree(
new Object[] {"this", "that", "other"}));This displays a primitive tree. The API for trees is vast, however—certainly one of the largest in Swing. It appears that you can do just about anything with trees, but more sophisticated tasks might require quite a bit of research and experimentation. Comment
Fortunately, there is a middle ground provided in the library: the “default” tree components, which generally do what you need. So most of the time you can use these components, and only in special cases will you need to delve in and understand trees more deeply. Comment
The following example uses the “default” tree components to display a tree in an applet. When you press the button, a new subtree is added under the currently selected node (if no node is selected, the root node is used): Comment
//: c13:Trees.java
// Simple Swing tree example. Trees can
// be made vastly more complex than this.
// <applet code=Trees
// width=250 height=250></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.tree.*;
import com.bruceeckel.swing.*;
// Takes an array of Strings and makes the first
// element a node and the rest leaves:
class Branch {
DefaultMutableTreeNode r;
public Branch(String[] data) {
r = new DefaultMutableTreeNode(data[0]);
for(int i = 1; i < data.length; i++)
r.add(new DefaultMutableTreeNode(data[i]));
}
public DefaultMutableTreeNode node() {
return r;
}
}
public class Trees extends JApplet {
String[][] data = {
{ "Colors", "Red", "Blue", "Green" },
{ "Flavors", "Tart", "Sweet", "Bland" },
{ "Length", "Short", "Medium", "Long" },
{ "Volume", "High", "Medium", "Low" },
{ "Temperature", "High", "Medium", "Low" },
{ "Intensity", "High", "Medium", "Low" },
};
static int i = 0;
DefaultMutableTreeNode root, child, chosen;
JTree tree;
DefaultTreeModel model;
public void init() {
Container cp = getContentPane();
root = new DefaultMutableTreeNode("root");
tree = new JTree(root);
// Add it and make it take care of scrolling:
cp.add(new JScrollPane(tree),
BorderLayout.CENTER);
// Capture the tree's model:
model =(DefaultTreeModel)tree.getModel();
JButton test = new JButton("Press me");
test.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
if(i < data.length) {
child = new Branch(data[i++]).node();
// What's the last one you clicked?
chosen = (DefaultMutableTreeNode)
tree.getLastSelectedPathComponent();
if(chosen == null) chosen = root;
// The model will create the
// appropriate event. In response, the
// tree will update itself:
model.insertNodeInto(child, chosen, 0);
// This puts the new node on the
// currently chosen node.
}
}
});
// Change the button's colors:
test.setBackground(Color.blue);
test.setForeground(Color.white);
JPanel p = new JPanel();
p.add(test);
cp.add(p, BorderLayout.SOUTH);
}
public static void main(String[] args) {
Console.run(new Trees(), 250, 250);
}
} ///:~The first class, Branch, is a tool to take an array of String and build a DefaultMutableTreeNode with the first String as the root and the rest of the Strings in the array as leaves. Then node( ) can be called to produce the root of this “branch.” Comment
The Trees class contains a two-dimensional array of Strings from which Branches can be made and a static int i to count through this array. The DefaultMutableTreeNode objects hold the nodes, but the physical representation on screen is controlled by the JTree and its associated model, the DefaultTreeModel. Note that when the JTree is added to the applet, it is wrapped in a JScrollPane—this is all it takes to provide automatic scrolling. Comment
The JTree is controlled through its model. When you make a change to the model, the model generates an event that causes the JTree to perform any necessary updates to the visible representation of the tree. In init( ), the model is captured by calling getModel( ). When the button is pressed, a new “branch” is created. Then the currently selected component is found (or the root is used if nothing is selected) and the model’s insertNodeInto( ) method does all the work of changing the tree and causing it to be updated. Comment
An example like the one above may give you what you need in a tree. However, trees have the power to do just about anything you can imagine—everywhere you see the word “default” in the example above, you can substitute your own class to get different behavior. But beware: almost all of these classes have a large interface, so you could spend a lot of time struggling to understand the intricacies of trees. Despite this, it’s a good design and the alternatives are usually much worse. Comment
Like trees, tables in Swing are vast and powerful. They are primarily intended to be the popular “grid” interface to databases via Java Database Connectivity (JDBC, discussed in Chapter 15) and thus they have a tremendous amount of flexibility, which you pay for in complexity. There’s easily enough here to be the basis of a full-blown spreadsheet and could probably justify an entire book. However, it is also possible to create a relatively simple JTable if you understand the basics. Comment
The JTable controls how the data is displayed, but the TableModel controls the data itself. So to create a JTable you’ll typically create a TableModel first. You can fully implement the TableModel interface, but it’s usually simpler to inherit from the helper class AbstractTableModel:
//: c13:JTableDemo.java
// Simple demonstration of JTable.
// <applet code=Table
// width=350 height=200></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.table.*;
import javax.swing.event.*;
import com.bruceeckel.swing.*;
public class JTableDemo extends JApplet {
JTextArea txt = new JTextArea(4, 20);
// The TableModel controls all the data:
class DataModel extends AbstractTableModel {
Object[][] data = {
{"one", "two", "three", "four"},
{"five", "six", "seven", "eight"},
{"nine", "ten", "eleven", "twelve"},
};
// Prints data when table changes:
class TML implements TableModelListener {
public void tableChanged(TableModelEvent e){
txt.setText(""); // Clear it
for(int i = 0; i < data.length; i++) {
for(int j = 0; j < data[0].length; j++)
txt.append(data[i][j] + " ");
txt.append("\n");
}
}
}
public DataModel() {
addTableModelListener(new TML());
}
public int getColumnCount() {
return data[0].length;
}
public int getRowCount() {
return data.length;
}
public Object getValueAt(int row, int col) {
return data[row][col];
}
public void
setValueAt(Object val, int row, int col) {
data[row][col] = val;
// Indicate the change has happened:
fireTableDataChanged();
}
public boolean
isCellEditable(int row, int col) {
return true;
}
}
public void init() {
Container cp = getContentPane();
JTable table = new JTable(new DataModel());
cp.add(new JScrollPane(table));
cp.add(BorderLayout.SOUTH, txt);
}
public static void main(String[] args) {
Console.run(new JTableDemo(), 350, 200);
}
} ///:~DataModel contains an array of data, but you could also get the data from some other source such as a database. The constructor adds a TableModelListener that prints the array every time the table is changed. The rest of the methods follow the Beans naming convention, and are used by JTable when it wants to present the information in DataModel. AbstractTableModel provides default methods for setValueAt( ) and isCellEditable( ) that prevent changes to the data, so if you want to be able to edit the data, you must override these methods. Comment
Once you have a TableModel, you only need to hand it to the JTable constructor. All the details of displaying, editing, and updating will be taken care of for you. This example also puts the JTable in a JScrollPane. Comment
One of the very interesting aspects of Swing is the “Pluggable Look & Feel.” This allows your program to emulate the look and feel of various operating environments. You can even do all sorts of fancy things like dynamically changing the look and feel while the program is executing. However, you generally just want to do one of two things, either select the “cross platform” look and feel (which is Swing’s “metal”), or select the look and feel for the system you are currently on, so your Java program looks like it was created specifically for that system. The code to select either of these behaviors is quite simple—but you must execute it before you create any visual components, because the components will be made based on the current look and feel and will not be changed just because you happen to change the look and feel midway during the program (that process is more complicated and uncommon, and is relegated to Swing-specific books). Comment
Actually, if you want to use the cross-platform (“metal”) look and feel that is characteristic of Swing programs, you don’t have to do anything—it’s the default. But if you want instead to use the current operating environment’s look and feel, you just insert the following code, typically at the beginning of your main( ) but somehow before any components are added:
try {
UIManager.setLookAndFeel(UIManager.
getSystemLookAndFeelClassName());
} catch(Exception e) {}You don’t need anything in the catch clause because the UIManager will default to the cross-platform look and feel if your attempts to set up any of the alternatives fail. However, during debugging the exception can be quite useful so you may at least want to put a print statement in the catch clause. Comment
Here is a program that takes a command-line argument to select a look and feel, and shows how several different components look under the chosen look and feel:
//: c13:LookAndFeel.java
// Selecting different looks & feels.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;
public class LookAndFeel extends JFrame {
String[] choices = {
"eeny", "meeny", "minie", "moe", "toe", "you"
};
Component[] samples = {
new JButton("JButton"),
new JTextField("JTextField"),
new JLabel("JLabel"),
new JCheckBox("JCheckBox"),
new JRadioButton("Radio"),
new JComboBox(choices),
new JList(choices),
};
public LookAndFeel() {
super("Look And Feel");
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
for(int i = 0; i < samples.length; i++)
cp.add(samples[i]);
}
private static void usageError() {
System.out.println(
"Usage:LookAndFeel [cross|system|motif]");
System.exit(1);
}
public static void main(String[] args) {
if(args.length == 0) usageError();
if(args[0].equals("cross")) {
try {
UIManager.setLookAndFeel(UIManager.
getCrossPlatformLookAndFeelClassName());
} catch(Exception e) {
e.printStackTrace(System.err);
}
} else if(args[0].equals("system")) {
try {
UIManager.setLookAndFeel(UIManager.
getSystemLookAndFeelClassName());
} catch(Exception e) {
e.printStackTrace(System.err);
}
} else if(args[0].equals("motif")) {
try {
UIManager.setLookAndFeel("com.sun.java."+
"swing.plaf.motif.MotifLookAndFeel");
} catch(Exception e) {
e.printStackTrace(System.err);
}
} else usageError();
// Note the look & feel must be set before
// any components are created.
Console.run(new LookAndFeel(), 300, 200);
}
} ///:~You can see that one option is to explicitly specify a string for a look and feel, as seen with MotifLookAndFeel. However, that one and the default “metal” look and feel are the only ones that can legally be used on any platform; even though there are strings for Windows and Macintosh look and feels, those can only be used on their respective platforms (these are produced when you call getSystemLookAndFeelClassName( ) and you’re on that particular platform). Comment
It is also possible to create a custom look and feel package, for example, if you are building a framework for a company that wants a distinctive appearance. This is a big job and is far beyond the scope of this book (in fact, you’ll discover it is beyond the scope of many dedicated Swing books!). Comment
The JFC supports limited operations with the system clipboard (in the java.awt.datatransfer package). You can copy String objects to the clipboard as text, and you can paste text from the clipboard into String objects. Of course, the clipboard is designed to hold any type of data, but how this data is represented on the clipboard is up to the program doing the cutting and pasting. The Java clipboard API provides for extensibility through the concept of a “flavor.” When data comes off the clipboard, it has an associated set of flavors that it can be converted to (for example, a graph might be represented as a string of numbers or as an image) and you can see if that particular clipboard data supports the flavor you’re interested in. Comment
The following program is a simple demonstration of cut, copy, and paste with String data in a JTextArea. One thing you’ll notice is that the keyboard sequences you normally use for cutting, copying, and pasting also work. But if you look at any JTextField or JTextArea in any other program you’ll find that they also automatically support the clipboard key sequences. This example simply adds programmatic control of the clipboard, and you could use these techniques if you want to capture clipboard text into something other than a JTextComponent.
//: c13:CutAndPaste.java
// Using the clipboard.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.datatransfer.*;
import com.bruceeckel.swing.*;
public class CutAndPaste extends JFrame {
JMenuBar mb = new JMenuBar();
JMenu edit = new JMenu("Edit");
JMenuItem
cut = new JMenuItem("Cut"),
copy = new JMenuItem("Copy"),
paste = new JMenuItem("Paste");
JTextArea text = new JTextArea(20, 20);
Clipboard clipbd =
getToolkit().getSystemClipboard();
public CutAndPaste() {
cut.addActionListener(new CutL());
copy.addActionListener(new CopyL());
paste.addActionListener(new PasteL());
edit.add(cut);
edit.add(copy);
edit.add(paste);
mb.add(edit);
setJMenuBar(mb);
getContentPane().add(text);
}
class CopyL implements ActionListener {
public void actionPerformed(ActionEvent e) {
String selection = text.getSelectedText();
if (selection == null)
return;
StringSelection clipString =
new StringSelection(selection);
clipbd.setContents(clipString,clipString);
}
}
class CutL implements ActionListener {
public void actionPerformed(ActionEvent e) {
String selection = text.getSelectedText();
if (selection == null)
return;
StringSelection clipString =
new StringSelection(selection);
clipbd.setContents(clipString, clipString);
text.replaceRange("",
text.getSelectionStart(),
text.getSelectionEnd());
}
}
class PasteL implements ActionListener {
public void actionPerformed(ActionEvent e) {
Transferable clipData =
clipbd.getContents(CutAndPaste.this);
try {
String clipString =
(String)clipData.
getTransferData(
DataFlavor.stringFlavor);
text.replaceRange(clipString,
text.getSelectionStart(),
text.getSelectionEnd());
} catch(Exception ex) {
System.err.println("Not String flavor");
}
}
}
public static void main(String[] args) {
Console.run(new CutAndPaste(), 300, 200);
}
} ///:~The creation and addition of the menu and JTextArea should by now seem a pedestrian activity. What’s different is the creation of the Clipboard field clipbd, which is done through the Toolkit. Comment
All the action takes place in the listeners. The CopyL and CutL listeners are the same except for the last line of CutL, which erases the line that’s been copied. The special two lines are the creation of a StringSelection object from the String and the call to setContents( ) with this StringSelection. That’s all there is to putting a String on the clipboard. Comment
In PasteL, data is pulled off the clipboard using getContents( ). What comes back is a fairly anonymous Transferable object, and you don’t really know what it contains. One way to find out is to call getTransferDataFlavors( ), which returns an array of DataFlavor objects indicating which flavors are supported by this particular object. You can also ask it directly with isDataFlavorSupported( ), passing in the flavor you’re interested in. Here, however, the bold approach is taken: getTransferData( ) is called assuming that the contents supports the String flavor, and if it doesn’t the problem is sorted out in the exception handler. Comment
In the future you can expect more data flavors to be supported.
An important use of the JAR utility is to optimize applet loading. In Java 1.0, people tended to try to cram all their code into a single applet class so the client would need only a single server hit to download the applet code. Not only did this result in messy, hard to read (and maintain) programs, but the .class file was still uncompressed so downloading wasn’t as fast as it could have been. Comment
JAR files solve the problem by compressing all of your .class files into a single file that is downloaded by the browser. Now you can create the right design without worrying about how many .class files it will generate, and the user will get a much faster download time. Comment
Consider TicTacToe.java. It looks like a single class, but in fact it contains five inner classes, so that’s six in all. Once you’ve compiled the program, you package it into a JAR file with the line:
jar cf TicTacToe.jar *.class
This assumes that the only .class files in the current directory are the ones from TicTacToe.java (otherwise you’ll get extra baggage). Comment
Now you can create an HTML page with the new archive tag to indicate the name of the JAR file. Here is the tag using the old form of the HTML tag, as an illustration:
<head><title>TicTacToe Example Applet
</title></head>
<body>
<applet code=TicTacToe.class
archive=TicTacToe.jar
width=200 height=100>
</applet>
</body>You’ll need to put it into the new (messy, complicated) form shown earlier in the chapter in order to get it to work.Comment
Because GUI programming in Java has been an evolving technology with some very significant changes between Java 1.0/1.1 and the Swing library in Java 2, there have been some old programming idioms that have seeped through to examples that you might see given for Swing. In addition, Swing allows you to program in more and better ways than were allowed by the old models. In this section, some of these issues will be demonstrated by introducing and examining some programming idioms. Comment
One of the benefits of the Swing event model is flexibility. You can add and remove event behavior with single method calls. The following example demonstrates this:
//: c13:DynamicEvents.java
// You can change event behavior dynamically.
// Also shows multiple actions for an event.
// <applet code=DynamicEvents
// width=250 height=400></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;
public class DynamicEvents extends JApplet {
ArrayList v = new ArrayList();
int i = 0;
JButton
b1 = new JButton("Button1"),
b2 = new JButton("Button2");
JTextArea txt = new JTextArea();
class B implements ActionListener {
public void actionPerformed(ActionEvent e) {
txt.append("A button was pressed\n");
}
}
class CountListener implements ActionListener {
int index;
public CountListener(int i) { index = i; }
public void actionPerformed(ActionEvent e) {
txt.append("Counted Listener "+index+"\n");
}
}
class B1 implements ActionListener {
public void actionPerformed(ActionEvent e) {
txt.append("Button 1 pressed\n");
ActionListener a = new CountListener(i++);
v.add(a);
b2.addActionListener(a);
}
}
class B2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
txt.append("Button2 pressed\n");
int end = v.size() - 1;
if(end >= 0) {
b2.removeActionListener(
(ActionListener)v.get(end));
v.remove(end);
}
}
}
public void init() {
Container cp = getContentPane();
b1.addActionListener(new B());
b1.addActionListener(new B1());
b2.addActionListener(new B());
b2.addActionListener(new B2());
JPanel p = new JPanel();
p.add(b1);
p.add(b2);
cp.add(BorderLayout.NORTH, p);
cp.add(new JScrollPane(txt));
}
public static void main(String[] args) {
Console.run(new DynamicEvents(), 250, 400);
}
} ///:~The new twists in this example are: Comment
This kind of flexibility provides much greater power in your programming. Comment
You should notice that event listeners are not guaranteed to be called in the order they are added (although most implementations do in fact work that way). Comment
In general you’ll want to design your classes so that each one does “only one thing.” This is particularly important when user-interface code is concerned, since it’s easy to tie up “what you’re doing” with “how you’re displaying it.” This kind of coupling prevents code reuse. It’s much more desirable to separate your “business logic” from the GUI. This way, you can not only reuse the business logic more easily, it’s also easier to reuse the GUI.
Another issue is multitiered systems, where the “business objects” reside on a completely separate machine. This central location of the business rules allows changes to be instantly effective for all new transactions, and is thus a compelling way to set up a system. However, these business objects can be used in many different applications and so should not be tied to any particular mode of display. They should just perform the business operations and nothing more. Comment
The following example shows how easy it is to separate the business logic from the GUI code:
//: c13:Separation.java
// Separating GUI logic and business objects.
// <applet code=Separation
// width=250 height=150> </applet>
import javax.swing.*;
import java.awt.*;
import javax.swing.event.*;
import java.awt.event.*;
import java.applet.*;
import com.bruceeckel.swing.*;
class BusinessLogic {
private int modifier;
public BusinessLogic(int mod) {
modifier = mod;
}
public void setModifier(int mod) {
modifier = mod;
}
public int getModifier() {
return modifier;
}
// Some business operations:
public int calculation1(int arg) {
return arg * modifier;
}
public int calculation2(int arg) {
return arg + modifier;
}
}
public class Separation extends JApplet {
JTextField
t = new JTextField(15),
mod = new JTextField(15);
BusinessLogic bl = new BusinessLogic(2);
JButton
calc1 = new JButton("Calculation 1"),
calc2 = new JButton("Calculation 2");
static int getValue(JTextField tf) {
try {
return Integer.parseInt(tf.getText());
} catch(NumberFormatException e) {
return 0;
}
}
class Calc1L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText(Integer.toString(
bl.calculation1(getValue(t))));
}
}
class Calc2L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText(Integer.toString(
bl.calculation2(getValue(t))));
}
}
// If you want something to happen whenever
// a JTextField changes, add this listener:
class ModL implements DocumentListener {
public void changedUpdate(DocumentEvent e) {}
public void insertUpdate(DocumentEvent e) {
bl.setModifier(getValue(mod));
}
public void removeUpdate(DocumentEvent e) {
bl.setModifier(getValue(mod));
}
}
public void init() {
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
calc1.addActionListener(new Calc1L());
calc2.addActionListener(new Calc2L());
JPanel p1 = new JPanel();
p1.add(calc1);
p1.add(calc2);
cp.add(p1);
mod.getDocument().
addDocumentListener(new ModL());
JPanel p2 = new JPanel();
p2.add(new JLabel("Modifier:"));
p2.add(mod);
cp.add(p2);
}
public static void main(String[] args) {
Console.run(new Separation(), 250, 100);
}
} ///:~You can see that BusinessLogic is a straightforward class that performs its operations without even a hint that it might be used in a GUI environment. It just does its job. Comment
Separation keeps track of all the UI details, and it talks to BusinessLogic only through its public interface. All the operations are centered around getting information back and forth through the UI and the BusinessLogic object. So Separation, in turn, just does its job. Since Separation knows only that it’s talking to a BusinessLogic object (that is, it isn’t highly coupled), it could be massaged into talking to other types of objects without much trouble. Comment
Thinking in terms of separating UI from business logic also makes life easier when you’re adapting legacy code to work with Java. Comment
Inner classes, the Swing event model, and the fact that the old event model is still supported along with new library features that rely on old-style programming has added a new element of confusion to the code design process. Now there are even more different ways for people to write unpleasant code. Comment
Except in extenuating circumstances you can always use the simplest and clearest approach: listener classes (typically written as inner classes) to solve your event-handling needs. This is the form used in most of the examples in this chapter. Comment
By following this model you should be able to reduce the statements in your programs that say: “I wonder what caused this event.” Each piece of code is concerned with doing, not type-checking. This is the best way to write your code; not only is it easier to conceptualize, but much easier to read and maintain. Comment
In the Concurrency chapter, I suggested that you think carefully before making a class as an implementation of Runnable. Of course, if you must inherit from a class and you want to add threading behavior to the class, Runnable is the correct solution. The following example exploits this by making a Runnable JPanel class that paints different colors on itself. This application is set up to take values from the command line to determine how big the grid of colors is and how long to sleep( ) between color changes. By playing with these values you’ll discover some interesting and possibly inexplicable features of threads:
//: c13:ColorBoxes.java
// Using the Runnable interface.
// <applet code=ColorBoxes width=500 height=400>
// <param name=grid value="12">
// <param name=pause value="50">
// </applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
class CBox extends JPanel implements Runnable {
private Thread t;
private int pause;
private static final Color[] colors = {
Color.BLACK, Color.BLUE, Color.CYAN,
Color.DARK_GRAY, Color.GRAY, Color.GREEN,
Color.LIGHT_GRAY, Color.MAGENTA,
Color.ORANGE, Color.PINK, Color.RED,
Color.WHITE, Color.YELLOW
};
private Color cColor = newColor();
private static final Color newColor() {
return colors[
(int)(Math.random() * colors.length)
];
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(cColor);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
}
public CBox(int pause) {
this.pause = pause;
t = new Thread(this);
t.start();
}
public void run() {
while(true) {
cColor = newColor();
repaint();
try {
t.sleep(pause);
} catch(InterruptedException e) {
System.err.println("Interrupted");
}
}
}
}
public class ColorBoxes extends JApplet {
private boolean isApplet = true;
private int grid = 12;
private int pause = 50;
public void init() {
// Get parameters from Web page:
if (isApplet) {
String gsize = getParameter("grid");
if(gsize != null)
grid = Integer.parseInt(gsize);
String pse = getParameter("pause");
if(pse != null)
pause = Integer.parseInt(pse);
}
Container cp = getContentPane();
cp.setLayout(new GridLayout(grid, grid));
for (int i = 0; i < grid * grid; i++)
cp.add(new CBox(pause));
}
public static void main(String[] args) {
ColorBoxes applet = new ColorBoxes();
applet.isApplet = false;
if(args.length > 0)
applet.grid = Integer.parseInt(args[0]);
if(args.length > 1)
applet.pause = Integer.parseInt(args[1]);
Console.run(applet, 500, 400);
}
} ///:~ColorBoxes is the usual applet/application with an init( ) that sets up the GUI. This configures a GridLayout so that it has grid cells in each dimension. Then it adds the appropriate number of CBox objects to fill the grid, passing the pause value to each one. In main( ) you can see how pause and grid have default values that can be changed if you pass in command-line arguments, or by using applet parameters. Comment
CBox is where all the work takes place. This is inherited from JPanel and it implements the Runnable interface so each JPanel can also be a Thread. Remember that when you implement Runnable, you don’t make a Thread object, just a class that has a run( ) method. Thus, you must explicitly create a Thread object and hand the Runnable object to the constructor, then call start( ) (this happens in the constructor). In CBox this thread is called t. Comment
Notice the array colors, which is an enumeration of all the colors in class Color. This is used in newColor( ) to produce a randomly selected color. The current cell color is cColor. Comment
paintComponent( ) is quite simple—it just sets the color to cColor and fills the entire JPanel with that color. Comment
In run( ), you see the infinite loop that sets the cColor to a new random color and then calls repaint( ) to show it. Then the thread goes to sleep( ) for the amount of time specified on the command line. Comment
Precisely because this design is flexible and threading is tied to each JPanel element, you can experiment by making as many threads as you want. (In reality, there is a restriction imposed by the number of threads your JVM can comfortably handle.) Comment
This program also makes an interesting benchmark, since it can show dramatic performance differences between one JVM threading implementation and another. Comment
So far in this book you’ve seen how valuable Java is for creating reusable pieces of code. The “most reusable” unit of code has been the class, since it comprises a cohesive unit of characteristics (fields) and behaviors (methods) that can be reused either directly via composition or through inheritance. Comment
Inheritance and polymorphism are essential parts of object-oriented programming, but in the majority of cases when you’re putting together an application, what you really want is components that do exactly what you need. You’d like to drop these parts into your design like the electronic engineer puts together chips on a circuit board. It seems, too, that there should be some way to accelerate this “modular assembly” style of programming. Comment
“Visual programming” first became successful—very successful—with Microsoft’s Visual Basic (VB), followed by a second-generation design in Borland’s Delphi (the primary inspiration for the JavaBeans design). With these programming tools the components are represented visually, which makes sense since they usually display some kind of visual component such as a button or a text field. The visual representation, in fact, is often exactly the way the component will look in the running program. So part of the process of visual programming involves dragging a component from a palette and dropping it onto your form. The application builder tool writes code as you do this, and that code will cause the component to be created in the running program. Comment
Simply dropping the component onto a form is usually not enough to complete the program. Often, you must change the characteristics of a component, such as what color it is, what text is on it, what database it’s connected to, etc. Characteristics that can be modified at design time are referred to as properties. You can manipulate the properties of your component inside the application builder tool, and when you create the program this configuration data is saved so that it can be rejuvenated when the program is started. Comment
By now you’re probably used to the idea that an object is more than characteristics; it’s also a set of behaviors. At design-time, the behaviors of a visual component are partially represented by events, meaning “Here’s something that can happen to the component.” Ordinarily, you decide what you want to happen when an event occurs by tying code to that event. Comment
Here’s the critical part: the application builder tool uses reflection to dynamically interrogate the component and find out which properties and events the component supports. Once it knows what they are, it can display the properties and allow you to change those (saving the state when you build the program), and also display the events. In general, you do something like double-clicking on an event and the application builder tool creates a code body and ties it to that particular event. All you have to do at that point is write the code that executes when the event occurs. Comment
All this adds up to a lot of work that’s done for you by the application builder tool. As a result you can focus on what the program looks like and what it is supposed to do, and rely on the application builder tool to manage the connection details for you. The reason that visual programming tools have been so successful is that they dramatically speed up the process of building an application—certainly the user interface, but often other portions of the application as well. Comment
After the dust settles, then, a component is really just a block of code, typically embodied in a class. The key issue is the ability for the application builder tool to discover the properties and events for that component. To create a VB component, the programmer had to write a fairly complicated piece of code following certain conventions to expose the properties and events. Delphi was a second-generation visual programming tool and the language was actively designed around visual programming so it is much easier to create a visual component. However, Java has brought the creation of visual components to its most advanced state with JavaBeans, because a Bean is just a class. You don’t have to write any extra code or use special language extensions in order to make something a Bean. The only thing you need to do, in fact, is slightly modify the way that you name your methods. It is the method name that tells the application builder tool whether this is a property, an event, or just an ordinary method. Comment
In the Java documentation, this naming convention is mistakenly termed a “design pattern.” This is unfortunate, since design patterns (see Thinking in Patterns with Java, downloadable at www.BruceEckel.com) are challenging enough without this sort of confusion. It’s not a design pattern, it’s just a naming convention and it’s fairly simple:
Point 1 above answers a question about something you might have noticed when looking at older code vs. newer code: a number of method names have had small, apparently meaningless name changes. Now you can see that most of those changes had to do with adapting to the “get” and “set” naming conventions in order to make that particular component into a Bean. Comment
We can use these guidelines to create a simple Bean:
//: frogbean:Frog.java
// A trivial JavaBean.
package frogbean;
import java.awt.*;
import java.awt.event.*;
class Spots {}
public class Frog {
private int jumps;
private Color color;
private Spots spots;
private boolean jmpr;
public int getJumps() { return jumps; }
public void setJumps(int newJumps) {
jumps = newJumps;
}
public Color getColor() { return color; }
public void setColor(Color newColor) {
color = newColor;
}
public Spots getSpots() { return spots; }
public void setSpots(Spots newSpots) {
spots = newSpots;
}
public boolean isJumper() { return jmpr; }
public void setJumper(boolean j) { jmpr = j; }
public void addActionListener(
ActionListener l) {
//...
}
public void removeActionListener(
ActionListener l) {
// ...
}
public void addKeyListener(KeyListener l) {
// ...
}
public void removeKeyListener(KeyListener l) {
// ...
}
// An "ordinary" public method:
public void croak() {
System.out.println("Ribbet!");
}
} ///:~First, you can see that it’s just a class. Usually, all your fields will be private, and accessible only through methods. Following the naming convention, the properties are jumps, color, spots, and jumper (notice the case change of the first letter in the property name). Although the name of the internal identifier is the same as the name of the property in the first three cases, in jumper you can see that the property name does not force you to use any particular identifier for internal variables (or, indeed, to even have any internal variables for that property). Comment
The events this Bean handles are ActionEvent and KeyEvent, based on the naming of the “add” and “remove” methods for the associated listener. Finally, you can see that the ordinary method croak( ) is still part of the Bean simply because it’s a public method, not because it conforms to any naming scheme. Comment
One of the most critical parts of the Bean scheme occurs when you drag a Bean off a palette and plop it onto a form. The application builder tool must be able to create the Bean (which it can do if there’s a default constructor) and then, without access to the Bean’s source code, extract all the necessary information to create the property sheet and event handlers. Comment
Part of the solution is already evident from the end of Chapter 12: Java reflection allows all the methods of an anonymous class to be discovered. This is perfect for solving the Bean problem without requiring you to use any extra language keywords like those required in other visual programming languages. In fact, one of the prime reasons that reflection was added to Java was to support Beans (although reflection also supports object serialization and remote method invocation). So you might expect that the creator of the application builder tool would have to reflect each Bean and hunt through its methods to find the properties and events for that Bean. Comment
This is certainly possible, but the Java designers wanted to provide a standard tool, not only to make Beans simpler to use but also to provide a standard gateway to the creation of more complex Beans. This tool is the Introspector class, and the most important method in this class is the static getBeanInfo( ). You pass a Class reference to this method and it fully interrogates that class and returns a BeanInfo object that you can then dissect to find properties, methods, and events. Comment
Usually you won’t care about any of this—you’ll probably get most of your Beans off the shelf from vendors, and you don’t need to know all the magic that’s going on underneath. You’ll simply drag your Beans onto your form, then configure their properties and write handlers for the events you’re interested in. However, it’s an interesting and educational exercise to use the Introspector to display information about a Bean, so here’s a tool that does it:
//: c13:BeanDumper.java
// Introspecting a Bean.
// <applet code=BeanDumper width=600 height=500>
// </applet>
import java.beans.*;
import java.lang.reflect.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import com.bruceeckel.swing.*;
public class BeanDumper extends JApplet {
JTextField query =
new JTextField(20);
JTextArea results = new JTextArea();
public void prt(String s) {
results.append(s + "\n");
}
public void dump(Class bean){
results.setText("");
BeanInfo bi = null;
try {
bi = Introspector.getBeanInfo(
bean, java.lang.Object.class);
} catch(IntrospectionException e) {
prt("Couldn't introspect " +
bean.getName());
return;
}
PropertyDescriptor[] properties =
bi.getPropertyDescriptors();
for(int i = 0; i < properties.length; i++) {
Class p = properties[i].getPropertyType();
prt("Property type:\n " + p.getName() +
"Property name:\n " +
properties[i].getName());
Method readMethod =
properties[i].getReadMethod();
if(readMethod != null)
prt("Read method:\n " + readMethod);
Method writeMethod =
properties[i].getWriteMethod();
if(writeMethod != null)
prt("Write method:\n " + writeMethod);
prt("====================");
}
prt("Public methods:");
MethodDescriptor[] methods =
bi.getMethodDescriptors();
for(int i = 0; i < methods.length; i++)
prt(methods[i].getMethod().toString());
prt("======================");
prt("Event support:");
EventSetDescriptor[] events =
bi.getEventSetDescriptors();
for(int i = 0; i < events.length; i++) {
prt("Listener type:\n " +
events[i].getListenerType().getName());
Method[] lm =
events[i].getListenerMethods();
for(int j = 0; j < lm.length; j++)
prt("Listener method:\n " +
lm[j].getName());
MethodDescriptor[] lmd =
events[i].getListenerMethodDescriptors();
for(int j = 0; j < lmd.length; j++)
prt("Method descriptor:\n " +
lmd[j].getMethod());
Method addListener =
events[i].getAddListenerMethod();
prt("Add Listener Method:\n " +
addListener);
Method removeListener =
events[i].getRemoveListenerMethod();
prt("Remove Listener Method:\n " +
removeListener);
prt("====================");
}
}
class Dumper implements ActionListener {
public void actionPerformed(ActionEvent e) {
String name = query.getText();
Class c = null;
try {
c = Class.forName(name);
} catch(ClassNotFoundException ex) {
results.setText("Couldn't find " + name);
return;
}
dump(c);
}
}
public void init() {
Container cp = getContentPane();
JPanel p = new JPanel();
p.setLayout(new FlowLayout());
p.add(new JLabel("Qualified bean name:"));
p.add(query);
cp.add(BorderLayout.NORTH, p);
cp.add(new JScrollPane(results));
Dumper dmpr = new Dumper();
query.addActionListener(dmpr);
query.setText("frogbean.Frog");
// Force evaluation
dmpr.actionPerformed(
new ActionEvent(dmpr, 0, ""));
}
public static void main(String[] args) {
Console.run(new BeanDumper(), 600, 500);
}
} ///:~BeanDumper.dump( ) is the method that does all the work. First it tries to create a BeanInfo object, and if successful calls the methods of BeanInfo that produce information about properties, methods, and events. In Introspector.getBeanInfo( ), you’ll see there is a second argument. This tells the Introspector where to stop in the inheritance hierarchy. Here, it stops before it parses all the methods from Object, since we’re not interested in seeing those. Comment
For properties, getPropertyDescriptors( ) returns an array of PropertyDescriptors. For each PropertyDescriptor you can call getPropertyType( ) to find the class of object that is passed in and out via the property methods. Then, for each property you can get its pseudonym (extracted from the method names) with getName( ), the method for reading with getReadMethod( ), and the method for writing with getWriteMethod( ). These last two methods return a Method object that can actually be used to invoke the corresponding method on the object (this is part of reflection). Comment
For the public methods (including the property methods), getMethodDescriptors( ) returns an array of MethodDescriptors. For each one you can get the associated Method object and print its name. Comment
For the events, getEventSetDescriptors( ) returns an array of (what else?) EventSetDescriptors. Each of these can be queried to find out the class of the listener, the methods of that listener class, and the add- and remove-listener methods. The BeanDumper program prints out all of this information. Comment
Upon startup, the program forces the evaluation of frogbean.Frog. The output, after removing extra details that are unnecessary here, is:
class name: Frog Property type: Color Property name: color Read method: public Color getColor() Write method: public void setColor(Color) ==================== Property type: Spots Property name: spots Read method: public Spots getSpots() Write method: public void setSpots(Spots) ==================== Property type: boolean Property name: jumper Read method: public boolean isJumper() Write method: public void setJumper(boolean) ==================== Property type: int Property name: jumps Read method: public int getJumps() Write method: public void setJumps(int) ==================== Public methods: public void setJumps(int) public void croak() public void removeActionListener(ActionListener) public void addActionListener(ActionListener) public int getJumps() public void setColor(Color) public void setSpots(Spots) public void setJumper(boolean) public boolean isJumper() public void addKeyListener(KeyListener) public Color getColor() public void removeKeyListener(KeyListener) public Spots getSpots() ====================== Event support: Listener type: KeyListener Listener method: keyTyped Listener method: keyPressed Listener method: keyReleased Method descriptor: public void keyTyped(KeyEvent) Method descriptor: public void keyPressed(KeyEvent) Method descriptor: public void keyReleased(KeyEvent) Add Listener Method: public void addKeyListener(KeyListener) Remove Listener Method: public void removeKeyListener(KeyListener) ==================== Listener type: ActionListener Listener method: actionPerformed Method descriptor: public void actionPerformed(ActionEvent) Add Listener Method: public void addActionListener(ActionListener) Remove Listener Method: public void removeActionListener(ActionListener) ====================
This reveals most of what the Introspector sees as it produces a BeanInfo object from your Bean. You can see that the type of the property and its name are independent. Notice the lowercasing of the property name. (The only time this doesn’t occur is when the property name begins with more than one capital letter in a row.) And remember that the method names you’re seeing here (such as the read and write methods) are actually produced from a Method object that can be used to invoke the associated method on the object. Comment
The public method list includes the methods that are not associated with a property or event, such as croak( ), as well as those that are. These are all the methods that you can call programmatically for a Bean, and the application builder tool can choose to list all of these while you’re making method calls, to ease your task. Comment
Finally, you can see that the events are fully parsed out into the listener, its methods, and the add- and remove-listener methods. Basically, once you have the BeanInfo, you can find out everything of importance for the Bean. You can also call the methods for that Bean, even though you don’t have any other information except the object (again, a feature of reflection). Comment
This next example is slightly more sophisticated, albeit frivolous. It’s a JPanel that draws a little circle around the mouse whenever the mouse is moved. When you press the mouse, the word “Bang!” appears in the middle of the screen, and an action listener is fired. Comment
The properties you can change are the size of the circle as well as the color, size, and text of the word that is displayed when you press the mouse. A BangBean also has its own addActionListener( ) and removeActionListener( ) so you can attach your own listener that will be fired when the user clicks on the BangBean. You should be able to recognize the property and event support:
//: bangbean:BangBean.java
// A graphical Bean.
package bangbean;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
import com.bruceeckel.swing.*;
public class BangBean extends JPanel
implements Serializable {
protected int xm, ym;
protected int cSize = 20; // Circle size
protected String text = "Bang!";
protected int fontSize = 48;
protected Color tColor = Color.red;
protected ActionListener actionListener;
public BangBean() {
addMouseListener(new ML());
addMouseMotionListener(new MML());
}
public int getCircleSize() { return cSize; }
public void setCircleSize(int newSize) {
cSize = newSize;
}
public String getBangText() { return text; }
public void setBangText(String newText) {
text = newText;
}
public int getFontSize() { return fontSize; }
public void setFontSize(int newSize) {
fontSize = newSize;
}
public Color getTextColor() { return tColor; }
public void setTextColor(Color newColor) {
tColor = newColor;
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.black);
g.drawOval(xm - cSize/2, ym - cSize/2,
cSize, cSize);
}
// This is a unicast listener, which is
// the simplest form of listener management:
public void addActionListener (
ActionListener l)
throws TooManyListenersException {
if(actionListener != null)
throw new TooManyListenersException();
actionListener = l;
}
public void removeActionListener(
ActionListener l) {
actionListener = null;
}
class ML extends MouseAdapter {
public void mousePressed(MouseEvent e) {
Graphics g = getGraphics();
g.setColor(tColor);
g.setFont(
new Font(
"TimesRoman", Font.BOLD, fontSize));
int width =
g.getFontMetrics().stringWidth(text);
g.drawString(text,
(getSize().width - width) /2,
getSize().height/2);
g.dispose();
// Call the listener's method:
if(actionListener != null)
actionListener.actionPerformed(
new ActionEvent(BangBean.this,
ActionEvent.ACTION_PERFORMED, null));
}
}
class MML extends MouseMotionAdapter {
public void mouseMoved(MouseEvent e) {
xm = e.getX();
ym = e.getY();
repaint();
}
}
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
} ///:~The first thing you’ll notice is that BangBean implements the Serializable interface. This means that the application builder tool can “pickle” all the information for the BangBean using serialization after the program designer has adjusted the values of the properties. When the Bean is created as part of the running application, these “pickled” properties are restored so that you get exactly what you designed. Comment
You can see that all the fields are private, which is what you’ll usually do with a Bean—allow access only through methods, usually using the “property” scheme. Comment
When you look at the signature for addActionListener( ), you’ll see that it can throw a TooManyListenersException. This indicates that it is unicast, which means it notifies only one listener when the event occurs. Ordinarily, you’ll use multicast events so that many listeners can be notified of an event. However, that runs into issues that you won’t be ready for until the next chapter, so it will be revisited there (under the heading “JavaBeans revisited”). A unicast event sidesteps the problem. Comment
When you click the mouse, the text is put in the middle of the BangBean, and if the actionListener field is not null, its actionPerformed( ) is called, creating a new ActionEvent object in the process. Whenever the mouse is moved, its new coordinates are captured and the canvas is repainted (erasing any text that’s on the canvas, as you’ll see). Comment
Here is the BangBeanTest class to allow you to test the bean as either an applet or an application:
//: c13:BangBeanTest.java
// <applet code=BangBeanTest
// width=400 height=500></applet>
import bangbean.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;
public class BangBeanTest extends JApplet {
JTextField txt = new JTextField(20);
// During testing, report actions:
class BBL implements ActionListener {
int count = 0;
public void actionPerformed(ActionEvent e){
txt.setText("BangBean action "+ count++);
}
}
public void init() {
BangBean bb = new BangBean();
try {
bb.addActionListener(new BBL());
} catch(TooManyListenersException e) {
txt.setText("Too many listeners");
}
Container cp = getContentPane();
cp.add(bb);
cp.add(BorderLayout.SOUTH, txt);
}
public static void main(String[] args) {
Console.run(new BangBeanTest(), 400, 500);
}
} ///:~When a Bean is in a development environment, this class will not be used, but it’s helpful to provide a rapid testing method for each of your Beans. BangBeanTest places a BangBean within the applet, attaching a simple ActionListener to the BangBean to print an event count to the JTextField whenever an ActionEvent occurs. Usually, of course, the application builder tool would create most of the code that uses the Bean. Comment
When you run the BangBean through BeanDumper or put the BangBean inside a Bean-enabled development environment, you’ll notice that there are many more properties and actions than are evident from the above code. That’s because BangBean is inherited from JPanel, and JPanel is also Bean, so you’re seeing its properties and events as well. Comment
Whenever you create a Bean, you must assume that it will run in a multithreaded environment. This means that:
The first point is fairly easy to deal with, but the second point requires a little more thought. The previous version of BangBean.java ducked out of the multithreading question by ignoring the synchronized keyword and making the event unicast. Here it is modified to work in a multithreaded environment and to use multicasting for events:
//: c13:BangBean2.java
// You should write your Beans this way so they
// can run in a multithreaded environment.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
import com.bruceeckel.swing.*;
public class BangBean2 extends JPanel
implements Serializable {
private int xm, ym;
private int cSize = 20; // Circle size
private String text = "Bang!";
private int fontSize = 48;
private Color tColor = Color.red;
private ArrayList actionListeners =
new ArrayList();
public BangBean2() {
addMouseListener(new ML());
addMouseMotionListener(new MM());
}
public synchronized int getCircleSize() {
return cSize;
}
public synchronized void
setCircleSize(int newSize) {
cSize = newSize;
}
public synchronized String getBangText() {
return text;
}
public synchronized void
setBangText(String newText) {
text = newText;
}
public synchronized int getFontSize() {
return fontSize;
}
public synchronized void
setFontSize(int newSize) {
fontSize = newSize;
}
public synchronized Color getTextColor() {
return tColor;
}
public synchronized void
setTextColor(Color newColor) {
tColor = newColor;
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.black);
g.drawOval(xm - cSize/2, ym - cSize/2,
cSize, cSize);
}
// This is a multicast listener, which is
// more typically used than the unicast
// approach taken in BangBean.java:
public synchronized void
addActionListener(ActionListener l) {
actionListeners.add(l);
}
public synchronized void
removeActionListener(ActionListener l) {
actionListeners.remove(l);
}
// Notice this isn't synchronized:
public void notifyListeners() {
ActionEvent a =
new ActionEvent(BangBean2.this,
ActionEvent.ACTION_PERFORMED, null);
ArrayList lv = null;
// Make a shallow copy of the List in case
// someone adds a listener while we're
// calling listeners:
synchronized(this) {
lv = (ArrayList)actionListeners.clone();
}
// Call all the listener methods:
for(int i = 0; i < lv.size(); i++)
((ActionListener)lv.get(i))
.actionPerformed(a);
}
class ML extends MouseAdapter {
public void mousePressed(MouseEvent e) {
Graphics g = getGraphics();
g.setColor(tColor);
g.setFont(
new Font(
"TimesRoman", Font.BOLD, fontSize));
int width =
g.getFontMetrics().stringWidth(text);
g.drawString(text,
(getSize().width - width) /2,
getSize().height/2);
g.dispose();
notifyListeners();
}
}
class MM extends MouseMotionAdapter {
public void mouseMoved(MouseEvent e) {
xm = e.getX();
ym = e.getY();
repaint();
}
}
public static void main(String[] args) {
BangBean2 bb = new BangBean2();
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("ActionEvent" + e);
}
});
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("BangBean2 action");
}
});
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("More action");
}
});
Console.run(bb, 300, 300);
}
} ///:~Adding synchronized to the methods is an easy change. However, notice in addActionListener( ) and removeActionListener( ) that the ActionListeners are now added to and removed from an ArrayList, so you can have as many as you want. Comment
You can see that the method notifyListeners( ) is not synchronized. It can be called from more than one thread at a time. It’s also possible for addActionListener( ) or removeActionListener( ) to be called in the middle of a call to notifyListeners( ), which is a problem since it traverses the ArrayList actionListeners. To alleviate the problem, the ArrayList is cloned inside a synchronized clause and the clone is traversed (see Appendix A for details of cloning). This way the original ArrayList can be manipulated without impact on notifyListeners( ). Comment
The paintComponent( ) method is also not synchronized. Deciding whether to synchronize overridden methods is not as clear as when you’re just adding your own methods. In this example it turns out that paint( ) seems to work OK whether it’s synchronized or not. But the issues you must consider are:
The test code in main()has been modified from that seen in BangBeanTest to demonstrate the multicast ability of BangBean2 by adding extra listeners. Comment
Before you can bring a Bean into a Bean-enabled visual builder tool, it must be put into the standard Bean container, which is a JAR file that includes all the Bean classes as well as a “manifest” file that says “This is a Bean.” A manifest file is simply a text file that follows a particular form. For the BangBean, the manifest file looks like this (without the first and last lines):
//:! :BangBean.mf Manifest-Version: 1.0 Name: bangbean/BangBean.class Java-Bean: True ///:~
The first line indicates the version of the manifest scheme, which until further notice from Sun is 1.0. The second line (empty lines are ignored) names the BangBean.class file, and the third says, “It’s a Bean.” Without the third line, the program builder tool will not recognize the class as a Bean. Comment
The only tricky part is that you must make sure that you get the proper path in the “Name:” field. If you look back at BangBean.java, you’ll see it’s in package bangbean (and thus in a subdirectory called “bangbean” that’s off of the classpath), and the name in the manifest file must include this package information. In addition, you must place the manifest file in the directory above the root of your package path, which in this case means placing the file in the directory above the “bangbean” subdirectory. Then you must invoke jar from the same directory as the manifest file, as follows:
jar cfm BangBean.jar BangBean.mf bangbean
This assumes that you want the resulting JAR file to be named BangBean.jar and that you’ve put the manifest in a file called BangBean.mf. Comment
You might wonder “What about all the other classes that were generated when I compiled BangBean.java?” Well, they all ended up inside the bangbean subdirectory, and you’ll see that the last argument for the above jar command line is the bangbean subdirectory. When you give jar the name of a subdirectory, it packages that entire subdirectory into the jar file (including, in this case, the original BangBean.java source-code file—you might not choose to include the source with your own Beans). In addition, if you turn around and unpack the JAR file you’ve just created, you’ll discover that your manifest file isn’t inside, but that jar has created its own manifest file (based partly on yours) called MANIFEST.MF and placed it inside the subdirectory META-INF (for “meta-information”). If you open this manifest file you’ll also notice that digital signature information has been added by jar for each file, of the form:
Digest-Algorithms: SHA MD5 SHA-Digest: pDpEAG9NaeCx8aFtqPI4udSX/O0= MD5-Digest: O4NcS1hE3Smnzlp2hj6qeg==
In general, you don’t need to worry about any of this, and if you make changes you can just modify your original manifest file and reinvoke jar to create a new JAR file for your Bean. You can also add other Beans to the JAR file simply by adding their information to your manifest. Comment
One thing to notice is that you’ll probably want to put each Bean in its own subdirectory, since when you create a JAR file you hand the jar utility the name of a subdirectory and it puts everything in that subdirectory into the JAR file. You can see that both Frog and BangBean are in their own subdirectories. Comment
Once you have your Bean properly inside a JAR file you can bring it into a Beans-enabled program-builder environment. The way you do this varies from one tool to the next, but Sun provides a freely available test bed for JavaBeans in their “Beans Development Kit” (BDK) called the “beanbox.” (Download the BDK from java.sun.com/beans.) To place your Bean in the beanbox, copy the JAR file into the BDK’s “jars” subdirectory before you start up the beanbox. Comment
You can see how remarkably simple it is to make a Bean. But you aren’t limited to what you’ve seen here. The JavaBeans architecture provides a simple point of entry but can also scale to more complex situations. These situations are beyond the scope of this book, but they will be briefly introduced here. You can find more details at java.sun.com/beans. Comment
One place where you can add sophistication is with properties. The examples above have shown only single properties, but it’s also possible to represent multiple properties in an array. This is called an indexed property. You simply provide the appropriate methods (again following a naming convention for the method names) and the Introspector recognizes an indexed property so your application builder tool can respond appropriately. Comment
Properties can be bound, which means that they will notify other objects via a PropertyChangeEvent. The other objects can then choose to change themselves based on the change to the Bean. Comment
Properties can be constrained, which means that other objects can veto a change to that property if it is unacceptable. The other objects are notified using a PropertyChangeEvent, and they can throw a PropertyVetoException to prevent the change from happening and to restore the old values. Comment
You can also change the way your Bean is represented at design time: Comment
There’s another issue that couldn’t be addressed here. Whenever you create a Bean, you should expect that it will be run in a multithreaded environment. This means that you must understand the issues of threading, which will be introduced in Chapter 14. You’ll find a section there called “JavaBeans revisited” that will look at the problem and its solution. Comment
There are a number of books about JavaBeans; for example, JavaBeans by Elliotte Rusty Harold (IDG, 1998). Comment
Of all the libraries in Java, the GUI library has seen the most dramatic changes from Java 1.0 to Java 2. The Java 1.0 AWT was roundly criticized as being one of the worst designs seen, and while it would allow you to create portable programs, the resulting GUI was “equally mediocre on all platforms.” It was also limiting, awkward, and unpleasant to use compared with the native application development tools available on a particular platform. Comment
When Java 1.1 introduced the new event model and JavaBeans, the stage was set—now it was possible to create GUI components that could be easily dragged and dropped inside visual application builder tools. In addition, the design of the event model and Beans clearly shows strong consideration for ease of programming and maintainable code (something that was not evident in the 1.0 AWT). But it wasn’t until the JFC/Swing classes appeared that the job was finished. With the Swing components, cross-platform GUI programming can be a civilized experience. Comment
Actually, the only thing that’s missing is the application builder tool, and this is where the real revolution lies. Microsoft’s Visual Basic and Visual C++ require Microsoft’s application builder tools, as does Borland’s Delphi and C++ Builder. If you want the application builder tool to get better, you have to cross your fingers and hope the vendor will give you what you want. But Java is an open environment, and so not only does it allow for competing application builder environments, it encourages them. And for these tools to be taken seriously, they must support JavaBeans. This means a leveled playing field: if a better application builder tool comes along, you’re not tied to the one you’ve been using—you can pick up and move to the new one and increase your productivity. This kind of competitive environment for GUI application builder tools has not been seen before, and the resulting marketplace can generate only positive results for the productivity of the programmer. Comment
This chapter was meant only to give you an introduction to the power of Swing and to get you started so you could see how relatively simple it is to feel your way through the libraries. What you’ve seen so far will probably suffice for a good portion of your UI design needs. However, there’s a lot more to Swing—it’s intended to be a fully powered UI design tool kit. There’s probably a way to accomplish just about everything you can imagine.
If you don’t see what you need here, delve into the online documentation from Sun and search the Web, and if that’s not enough then find a dedicated Swing book—a good place to start is The JFC Swing Tutorial, by Walrath & Campione (Addison Wesley, 1999). Comment
Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for a small fee from www.BruceEckel.com.
[67] A variation on this is called “the principle of least astonishment,” which essentially says: “don’t surprise the user.”
[68] This is an example of the design pattern called the template method.
[69] It is assumed that the reader is familiar with the basics of HTML. It’s not too hard to figure out, and there are lots of books and resources.
[70] This page—in particular, the ‘clsid’ portion—seemed to work fine with both JDK1.2.2 and JDK1.3 rc-1. However, you may find that you have to change the tag sometime in the future. Details can be found at java.sun.com.
[71] In my opinion. And after you learn about Swing, you won’t want to waste your time on the earlier stuff.
[72] As described earlier, “Frame” was already taken by the AWT, so Swing uses JFrame.
[73] This will make sense after you’ve read further in this chapter. First, make the reference JApplet a static member of the class (instead of a local variable of main( )), and then call applet.stop( ) and applet.destroy( ) inside WindowAdapter.windowClosing( ) before you call System.exit( ).
[74] There is no MouseMotionEvent even though it seems like there ought to be. Clicking and motion is combined into MouseEvent, so this second appearance of MouseEvent in the table is not an error.
[75] In Java 1.0/1.1 you could not usefully inherit from the button object. This was only one of numerous fundamental design flaws.