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Example Two: SwingApplication

Topics illustrated in this example:

• Look and Feel
• Setting Up Buttons and Labels
• Handling Events
• Adding Borders Around Components

Let's look at another simple program, SwingApplication. Each time the user clicks the button (JButton), the label (JLabel) is updated.

Look and Feel

The following figure shows three views of a GUI that uses Swing components. Each picture shows the same program, SimpleExample, but with a different look and feel.

Java look and feel

CDE/Motif look and feel

Windows look and feel

Swing allows you to specify which look and feel your program uses--Java look and feel, CDE/Motif look and feel, Windows look and feel, and so on. The code in boldface type in the following snippet shows you how SwingApplication specifies its look and feel:

public static void main(String[] args) {
    try {
        UIManager.setLookAndFeel(
            UIManager.getCrossPlatformLookAndFeelClassName());
    } catch (Exception e) { }
    ...// Create and show the GUI...
}

The preceding code essentially says, "I don't care whether the user has chosen a look and feel-use the cross-platform look and feel (the Java look and feel)."

Setting Up Buttons and Labels

Like most GUIs, the SwingApplication GUI contains a button and a label. (Unlike most GUIs, that's about all that SwingApplication contains.) Here's the code that initializes the button:

JButton button = new JButton("I'm a Swing button!");
button.setMnemonic('i'); 
button.addActionListener(...create an action listener...);

The first line creates the button. The second sets the letter "i" as the mnemonic that the user can use to simulate a click of the button. For example, in the Java look and feel, typing Alt-i results in a button click. The third line registers an event handler for the button click, as discussed later in this section.

Here's the code that initializes and manipulates the label:

...// where instance variables are declared:
private static String labelPrefix = "Number of button clicks: ";
private int numClicks = 0;

...// in GUI initialization code:
final JLabel label = new JLabel(labelPrefix + "0    ");
...
label.setLabelFor(button);

...// in the event handler for button clicks:
label.setText(labelPrefix + numClicks);

The preceding code is pretty straightforward, except for the line that invokes the setLabelFor method. That code exists solely to hint to assistive technologies that the label describes the button.

Now that you know how to set up buttons, you also know how to set up check boxes and radio buttons, as they all inherit from the AbstractButton class. Check boxes are similar to radio buttons, but by convention their selection models are different. Any number of check boxes in a group-none, some, or all-can be selected. On the other hand, only one button can be selected from a group of radio buttons. The following figures show screenshots of two programs that use check boxes and radio buttons.

Caption: As you'd expect, the CheckBoxDemo application shows the use of check boxes, and the RadioButtonDemo application shows the use of radio buttons.

You'll get a chance to take a closer look at radio buttons in the section Example Five: VoteDialog.

Handling Events

Every time the user types a character or pushes a mouse button, an event occurs. Any object can be notified of the event. All the object has to do is implement the appropriate interface and be registered as an event listener on the appropriate event source.

How to Implement an Event Handler

Every event handler requires three pieces of code:

1. In the declaration for the event handler class, one line of code specifies that the class either implements a listener interface or extends a class that implements a listener interface. For example:

   public class MyClass implements ActionListener { 
   

2. Another line of code registers an instance of the event handler class as a listener on one or more components. For example:

   someComponent.addActionListener(instanceOfMyClass);
   

3. In the event handler class, a few lines of code implement the methods in the listener interface. For example:

   public void actionPerformed(ActionEvent e) { 
       ...//code that reacts to the action... 
   } 
   

Event handlers can be instances of any class. Often an event handler that has only a few lines of code is implemented using an anonymous inner class--an unnamed class defined inside of another class. Anonymous inner classes can be confusing at first, but once you're used to them, they make the code clearer by keeping the implementation of an event handler close to where the event handler is registered.

SwingApplication has two event handlers. One handles window closing (window events); the other handles button clicks (action events). We've already seen the window-closing code. Here is the code that handles button clicks in the SwingApplication:

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        numClicks++;
        label.setText(labelPrefix + numClicks);
    }
});

In general, to detect when the user clicks an on-screen button (or does the keyboard equivalent), a program must have an object that implements the ActionListener interface. The program must register this object as an action listener on the button (the event source), using the addActionListener method. When the user clicks the on-screen button, the button fires an action event. This results in the invocation of the action listener's actionPerformed method (the only method in the ActionListener interface). The single argument to the method is an ActionEvent object that gives information about the event and its source.

Swing components can generate many kinds of events. The following table lists a few examples.

Examples of Events and Their Associated Event Listeners
Act that Results in the Event	Listener Type
User clicks a button, presses Return while typing in a text field, or chooses a menu item	ActionListener
User closes a frame (main window)	WindowListener
User presses a mouse button while the cursor is over a component	MouseListener
User moves the mouse over a component	MouseMotionListener
Component becomes visible	ComponentListener
Component gets the keyboard focus	FocusListener
Table or list selection changes	ListSelectionListener

To learn more about how to detect events from a particular component, refer to each component's how-to section in Using Swing Components.

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Note: Event-handling code executes in an single thread, the event-dispatching thread. This ensures that each event handler finishes execution before the next one executes. For instance, the actionPerformed method in the preceding example executes in the event-dispatching thread. Painting code also executes in the event-dispatching thread. Therefore, while the actionPerformed method is executing, the program's GUI is frozen--it won't repaint or respond to mouse clicks. See the section Threads and Swingfor more information.

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Adding Borders Around Components

If you take another look at the snapshot of SwingApplication, you'll notice that there is extra space surrounding the JPanel on all four edges.

Here is the code that adds a border to the panel:

pane.setBorder(BorderFactory.createEmptyBorder(
                                30, //top
                                30, //left
                                10, //bottom
                                30) //right
                                );

This border simply provides some empty space around the panel's contents--30 extra pixels on the top, left, and right, and 10 extra pixels on the bottom. Borders are a feature that JPanel inherits from the JComponent class.

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