Example One: Your First Swing Program - Java Tutorial 5.0 Ó¢ÎÄ°æ

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Example One: Your First Swing Program

This is the first of several sections that teach Swing basics by looking at example code. This section examines the code for a simple program, HelloWorldSwing. The examples in the following sections will become progressively more difficult as we introduce and explain more features.

Here's a snapshot of the HelloWorldSwing program:

And here's the full code for HelloWorldSwing:

import javax.swing.*;        

public class HelloWorldSwing {
    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Make sure we have nice window decorations.
        JFrame.setDefaultLookAndFeelDecorated(true);

        //Create and set up the window.
        JFrame frame = new JFrame("HelloWorldSwing");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Add the ubiquitous "Hello World" label.
        JLabel label = new JLabel("Hello World");
        frame.getContentPane().add(label);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

This is one of the simplest Swing applications you can write. It doesn’t do much, but the code demonstrates the basic code in every Swing program:

1. Import the pertinent packages.
2. Set up a top-level container.
3. Display the container.
4. Be thread-safe.

The first line imports the main Swing package:

import javax.swing.*;

This is the only package that HelloWorldSwing needs. However, most Swing programs also need to import two AWT packages:

import java.awt.*;
import java.awt.event.*;

These packages are required because Swing components use the AWT infrastructure, including the AWT event model. The event model governs how a component reacts to events such as button clicks and mouse motion. You'll learn more about events in the upcoming section Handling Events.

Every program with a Swing GUI must have at least one top-level Swing container. A top-level Swing container provides the support Swing components need for painting and event handling. There are three commonly used top-level Swing containers: JFrame, JDialog, and (for applets) JApplet. Each JFrame object implements a single main window, and each JDialog implements a secondary window (a window dependent on another window). Each JApplet object implements an applet’s display area within a browser window. (JApplet is covered in How to Make Applets.)

The HelloWorldSwing example has only one top-level container, a JFrame. Imple­mented as an instance of the JFrame class, a frame is a window that, by default, has dec­orations such as a border, a title, and buttons for iconifying and closing the window. Applications with a GUI typically use at least one frame.

Here is the code that sets up and shows the frame:

JFrame.setDefaultLookAndFeelDecorated(true);
JFrame frame = new JFrame("HelloWorldSwing");
...
frame.pack();
frame.setVisible(true);

---

Note: The following line of code applies decorative borders and window titles to frames. However, it works only as of v1.4. If you’re using an earlier version, you’ll need to comment out this code.

JFrame.setDefaultLookAndFeelDecorated(true);

---

With the exception of top-level containers, such as JFrame, all Swing components descend from the JComponent class. HelloWorldSwing uses a JComponent descendant called JLabel, which displays the text Hello World. These two lines of code construct and then add the JLabel component to the frame:

JLabel label = new JLabel("Hello World");
frame.getContentPane().add(label);

Note that the label is added to the frame’s content pane instead of to the frame itself. Every top-level container has a content pane that contains, directly or indirectly, all the visible components (except for menus and window decorations) in the top-level container.

---

Version Note: We anticipate that in v1.5 invoking add on a top-level container will have the same effect as invoking it on the top-level container’s content pane.

---

To make the program exit when the Close button is clicked, we include this code:

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

---

Version Note: In older programs, instead of a call to setDefaultCloseOperation, you might see code like the following:

frame.addWindowListener(new WindowAdapter() {
    public void windowClosing(WindowEvent e) {
        System.exit(0);
    }
});

It still works, but unnecessarily adds a class, which bloats your program. You can find further information on window events in How to Make Frames (Main Windows)

---

The final bit of code in HelloWorldSwing—-and in all of our examples—-looks like this:

javax.swing.SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        /* create and show the GUI */ 
    }
});

You can copy this code and use it as-is. It might look daunting, but we recommend it because it ensures that the GUI won’t have a thread-safety problem that could break the UI before it even appears onscreen. For more information, you can read How to Use Threads.

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