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// Change the state of Beta each time that Alpha is pressed if(jbtnBisEnabled()) { jlabsetText("Alpha pressed Beta is disabled"); jbtnBsetEnabled(false); } else { jlabsetText("Alpha pressed Beta is enabled"); jbtnBsetEnabled(true); } } else if(aegetSource() == jbtnB) jlabsetText("Beta pressed"); }
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Many programmers like this approach better than using the action command string because it avoids the overhead of a string comparison Of course, it does imply that the handler has access to the original component reference This may not always be convenient, or possible In these cases, the following alternative may be a good choice In the example, the ButtonDemo class implemented the ActionListener interface, supplying the actionPerformed( ) method While there is nothing wrong with this, it is not the only way to handle events Two other approaches are commonly used First, you can implement separate listener classes Thus, different classes could handle different events and these classes would be separate from the main class of the application Second, you can implement listeners through the use of anonymous inner classes Anonymous inner classes are inner classes that don t have a name Instead, an instance of the class is simply generated on the fly as needed Anonymous inner classes make implementing some types of event handlers much easier For instance, the action event handlers for jbtnA in the preceding example could be implemented using an anonymous inner class, as shown here:
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jbtnAaddActionListener(new ActionListener() { public void actionPerformed(ActionEvent ae) { if(jbtnBisEnabled()) { jlabsetText("Alpha pressed Beta is disabled"); jbtnBsetEnabled(false); } else { jlabsetText("Alpha pressed Beta is enabled"); jbtnBsetEnabled(true); } } });
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In this approach, an anonymous inner class is created that implements the ActionListener interface Pay special attention to the syntax The body of the inner class begins after the { that follows new ActionListener( ) Also notice that the call to addActionListener( ) ends with a ) and a ; just like normal The same basic syntax and approach is used to create an anonymous inner class for any event handler Of course, for different events, you specify different event listeners and implement different methods One advantage to using an anonymous inner class is that the component that invokes the class s methods is already known There is no need to call getActionCommand( ), for example, to determine what button generated the event because each implementation of actionPerformed( ) is associated with only one button, the one that generated the event
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Here is how the preceding example program looks when reworked to use anonymous inner classes to handle button action events
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// Use anonymous inner classes to handle JButton // action events Notice that ButtonDemo no longer // implements ActionListener import javaawt*; import javaawtevent*; import javaxswing*; class ButtonDemo { JLabel jlab; JButton jbtnA; JButton jbtnB; ButtonDemo() { // Create a new JFrame container JFrame jfrm = new JFrame("A Button Example"); // Set the layout manager to FlowLayout jfrmsetLayout(new FlowLayout()); // Give the frame an initial size jfrmsetSize(220, 90); // Terminate the program when the user closes the application jfrmsetDefaultCloseOperation(JFrameEXIT_ON_CLOSE); // Create a label jlab = new JLabel("Press a button"); // Make two buttons jbtnA = new JButton("Alpha"); jbtnB = new JButton("Beta"); // Use anonymous inner classes to handle button events jbtnAaddActionListener(new ActionListener() { public void actionPerformed(ActionEvent ae) { if(jbtnBisEnabled()) { jlabsetText("Alpha pressed Beta is disabled"); jbtnBsetEnabled(false); } else { jlabsetText("Alpha pressed Beta is enabled"); jbtnBsetEnabled(true); } } });
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jbtnBaddActionListener(new ActionListener() { public void actionPerformed(ActionEvent ae) { jlabsetText("Beta pressed"); } }); // Add the buttons and label to the content pane jfrmadd(jbtnA); jfrmadd(jbtnB); jfrmadd(jlab); // Display the frame jfrmsetVisible(true); } public static void main(String args[]) { // Create the frame on the event dispatching thread SwingUtilitiesinvokeLater(new Runnable() { public void run() { new ButtonDemo(); } }); } }
This program is functionally equivalent to the first version The difference is that now, each button is linked with its own action event handler There is no need for ButtonDemo to implement ActionListener or to use getActionCommand( ) to determine which button was pressed One last point: You can create a two-state button by using JToggleButton See Create a Toggle Button
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