Creating a Frame | Core Java(TM) 2, Volume I--Fundamentals (7th Edition) (Core Series) (Core Series)

A top-level window (that is, a window that is not contained inside another window) is called a frame in Java. The AWT library has a class, called Frame, for this top level. The Swing version of this class is called JFrame and extends the Frame class. The JFrame is one of the few Swing components that is not painted on a canvas. Thus, the decorations (buttons, title bar, icons, and so on) are drawn by the user's windowing system, not by Swing.

CAUTION

Most Swing component classes start with a "J": JButton, JFrame, and so on. There are classes such as Button and Frame, but they are AWT components. If you accidentally omit a "J", your program may still compile and run, but the mixture of Swing and AWT components can lead to visual and behavioral inconsistencies.

In this section, we go over the most common methods for working with a Swing JFrame. Example 7-1 lists a simple program that displays an empty frame on the screen, as illustrated in Figure 7-4.

Example 7-1. SimpleFrameTest.java

  1. import javax.swing.*;  2.  3. public class SimpleFrameTest  4. {  5.    public static void main(String[] args)  6.    {  7.       SimpleFrame frame = new SimpleFrame();  8.       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  9.       frame.setVisible(true); 10.    } 11. } 12. 13. class SimpleFrame extends JFrame 14. { 15.    public SimpleFrame() 16.    { 17.       setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); 18.    } 19. 20.    public static final int DEFAULT_WIDTH = 300; 21.    public static final int DEFAULT_HEIGHT = 200; 22. }

Figure 7-4. The simplest visible frame

Let's work through this program, line by line.

The Swing classes are placed in the javax.swing package. The package name javax indicates a Java extension package, not a core package. The Swing classes are indeed an extension to Java 1.1. Because the Swing classes were not made a part of the core hierarchy, it is possible to load the Swing classes into a Java 1.1-compatible browser. (The security manager of the browser does not allow adding any packages that start with "java.".) On the Java 2 platform, the Swing package is no longer an extension but is instead part of the core hierarchy. Any Java implementation that is compatible with Java 2 must supply the Swing classes. Nevertheless, the javax name remains, for compatibility with Java 1.1 code. (Actually, the Swing package started out as com.sun.java.swing, then was briefly moved to java.awt.swing during early Java 2 beta versions, then went back to com.sun.java.swing in late Java 2 beta versions, and after howls of protest by Java programmers, found its final resting place in javax.swing.)

By default, a frame has a rather useless size of 0 x 0 pixels. We define a subclass SimpleFrame whose constructor sets the size to 300 x 200 pixels. In the main method of the SimpleFrameTest class, we start out by constructing a SimpleFrame object.

Next, we define what should happen when the user closes this frame. For this particular program, we want the program to exit. To select this behavior, we use the statement

 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

In other programs with multiple frames, you would not want the program to exit just because the user closes one of the frames. By default, a frame is hidden when the user closes it, but the program does not terminate.

Simply constructing a frame does not automatically display it. Frames start their life invisible. That gives the programmer the chance to add components into the frame before showing it for the first time. To show the frame, the main method calls the setVisible method of the frame.

Afterwards, the main method exits. Note that exiting main does not terminate the program, just the main thread. Showing the frame activates a user interface thread that keeps the program alive.

NOTE

Before JDK 5.0, it was possible to use the show method that the JFrame class inherits from the superclass Window. The Window class has a superclass Component that also has a show method. The Component.show method was deprecated in JDK 1.2. You are supposed to call setVisible(true) instead if you want to show a component. However, until JDK 1.4, the Window.show method was not deprecated. In fact, it was quite useful, making the window visible and bringing it to the front. Sadly, that benefit was lost on the deprecation police, and JDK 5.0 deprecates the show method for windows as well.

The running program is shown in Figure 7-4 on page 249 it is a truly boring top-level window. As you can see in the figure, the title bar and surrounding decorations, such as resize corners, are drawn by the operating system and not the Swing library. If you run the same program in X Windows, the frame decorations are different. The Swing library draws everything inside the frame. In this program, it just fills the frame with a default background color.

NOTE

As of JDK 1.4, you can turn off all frame decorations by calling frame.setUndecorated(true).

NOTE

In the preceding example we wrote two classes, one to define a frame class and one that contains a main method that creates and shows a frame object. You will frequently see programs in which the main method is opportunistically tossed into a convenient class, like this:

 class SimpleFrame extends JFrame {    public static void main(String[] args)    {       SimpleFrame frame = new SimpleFrame();       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);       frame.show();    }    public SimpleFrame()    {       setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);    }    public static final int DEFAULT_WIDTH = 300;    public static final int DEFAULT_HEIGHT = 200; }

Using the main method of the frame class for the code that launches the program is simpler in one sense. You do not have to introduce another auxiliary class. However, quite a few programmers find this code style a bit confusing. Therefore, we prefer to separate out the class that launches the program from the classes that define the user interface.