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Here is a bare-bones applet that illustrates event handling. It contains a single button that beeps when you click it.
This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.You can find the entire program in
Beeper.java
. Here's the code that implements the event handling for the button:Isn't that simple? Thepublic class Beeper ... implements ActionListener { ... //where initialization occurs: button.addActionListener(this); ... public void actionPerformed(ActionEvent e) { ...//Make a beep sound... } }Beeper
class implements theActionListener
interface, which contains one method:actionPerformed
. SinceBeeper
implementsActionListener
, aBeeper
object can register as a listener for the action events that buttons fire. Once theBeeper
has been registered using theButton
addActionListener
method, theBeeper
'sactionPerformed
method is called every time the button is clicked.
The event model, which you saw at its simplest in the above example, is quite powerful and flexible. Any number of event listener objects can listen for all kinds of events from any number of event source objects. For example, a program might create one listener per event source. Or a program might have a single listener for all events from all sources. A program can even have more than one listener for a single kind of event from a single event source.The following applet gives an example of using multiple listeners per object. The applet contains two event sources (
JButton
instances) and two event listeners. One of the event listeners (an instance of a class calledMultiListener
) listens for events from both buttons. When it receives an event, it adds the event's "action command" (the text on the button's label) to the top text area. The second event listener (an instance of a class calledEavesdropper
) listens for events on only one of the buttons. When it receives an event, it adds the action command to the bottom text area.
This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.You can find the entire program in
MultiListener.java
. Here's the code that implements the event handling for the button:In the above code, bothpublic class MultiListener ... implements ActionListener { ... //where initialization occurs: button1.addActionListener(this); button2.addActionListener(this); button2.addActionListener(new Eavesdropper(bottomTextArea)); } public void actionPerformed(ActionEvent e) { topTextArea.append(e.getActionCommand() + newline); } } class Eavesdropper implements ActionListener { ... public void actionPerformed(ActionEvent e) { myTextArea.append(e.getActionCommand() + newline); } }MultiListener
andEavesdropper
implement theActionListener
interface and register as action listeners using theJButton
addActionListener
method. Both classes' implementations of theactionPerformed
method are similar: they simply add the event's action command to a text area.
So far, the only kind of event you've seen has been action events. Let's take a look at a program that handles another kind of event: mouse events.The following applet displays a rectangle-edged area and a text area. Whenever a mouse event -- a click, press, release, enter, or exit -- occurs on either the rectangle-edged area (
BlankArea
MouseEventDemo), the text area displays a string describing the event.
This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.You can find the entire program in
MouseEventDemo.java
andBlankArea.java
. Here's the code that implements the event handling:You'll see the code explained in How to Write a Mouse Listener, later in this section.public class MouseEventDemo ... implements MouseListener { ... //where initialization occurs: //Register for mouse events on blankArea and applet blankArea.addMouseListener(this); addMouseListener(this); } public void mousePressed(MouseEvent e) { saySomething("Mouse pressed; # of clicks: " + e.getClickCount(), e); } public void mouseReleased(MouseEvent e) { saySomething("Mouse released; # of clicks: " + e.getClickCount(), e); } public void mouseEntered(MouseEvent e) { saySomething("Mouse entered", e); } public void mouseExited(MouseEvent e) { saySomething("Mouse exited", e); } public void mouseClicked(MouseEvent e) { saySomething("Mouse clicked (# of clicks: " + e.getClickCount() + ")", e); } void saySomething(String eventDescription, MouseEvent e) { textArea.append(eventDescription + " detected on " + e.getComponent().getClass().getName() + "." + newline); } }
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