free visual basic barcode generator Commonly Used Event Listeners in Java

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Commonly Used Event Listeners
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You need to perform two steps in order to use an event listener First, you ll need to implement the appropriate event listener interface You ll see how this is done in examples throughout the rest of this chapter Second, you need to associate the listener with a GUI element Let s say you created a push button using the following statement:
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JButton bt = new JButton("Copy");
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A push button GUI element requires an ActionListener interface, so you ll need to implement that ActionListener interface in the class that creates the push button You associate the push button to the ActionListener interface by calling the addActionListener() method that is defined by the JButton class The addActionListener() method requires that you pass it the this keyword to reference the action event This is illustrated in the following statement:
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btaddActionListener(this);
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The ActionListener detects when the push button is selected by the user and then calls the actionPerformed() method The actionPerformed() method is a method you define in your program that reacts to the event The
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CHAPTER 12 Graphical User Interface
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actionPerformed() method requires one argument a reference to the event, as shown here:
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actionPerformed(actionEvent event)
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Statements within the method can use the event to determine the proper course of action to take
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Reading a Push Button
As mentioned in the previous section, you ll need to implement the ActionListener in order for your program to respond to the push button GUI element The following example shows how to react to the selection of a push button This example displays two text areas and one push button called Copy The addActionListener() method is called for the push button to associate the push button with the action listener If you fail to call the addActionListener() method, your program won t be able to react to the selection of a push button The push button and the text areas are then added to the content pane through a call to the add() method; then the content pane is placed in the container through a call to the setContentPane() method The actionPerformed() method is defined next Statements within this method are executed when the user clicks the push button This example causes the highlighted text in the first text area to be copied to the second text area when the user clicks the push button Figure 12-12 shows the window when this example is run
Figure 12-12
The second text area contains selected text copied from the first text area when the Copy push button is clicked
The actionPerformed() method is called whenever any push button is clicked Although this example shows one push button, typically you ll have multiple push buttons in the window of your application You determine which push button was clicked by calling the eventgetSource() method The
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eventgetSource() method returns the reference of the push button that was selected by the user You can compare this reference using an if statement to reference each push button on the screen to determine which push button was clicked You can also determine the kind of action that was performed by calling the eventgetActionCommand() method Try displaying the action that is returned by the eventgetActionCommand() method and see the kinds of actions that occur for the push button
import javaxswing*; import javaawt*; import javaawtevent*; public class Demo { public static void main(String[] args) { Window win = new Window(); } } class Window extends JFrame implements ActionListener { JTextArea ta1 = new JTextArea("default text", 2, 25); JTextArea ta2 = new JTextArea(2, 25); JButton bt1 = new JButton("Copy"); public Window () { super ("Hello world!"); setSize(400,200); setDefaultCloseOperation(JFrameEXIT_ON_CLOSE); setVisible(true); Container ca = getContentPane(); casetBackground(ColorlightGray); FlowLayout flm = new FlowLayout(); casetLayout(flm); bt1addActionListener(this); caadd(bt1); caadd(ta1); caadd(ta2); setContentPane(ca); } public void actionPerformed(ActionEvent event) { ta2setText(ta1getSelectedText()); } }
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