Named inner classes for listeners in Java

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/* * Simple3java - an example of handling events * For this example, we will use inner member classes to * implement an ActionListener for each button This approach * can avoid some of the code clutter that anonymous classes * can sometimes cause It also concentrates the action code * all in one place, and allows synonyms */ import javaawt*; import javaawtevent*; import javaxswing*; public class Simple3 extends JPanel { private static JFrame frame; // static so main can use it
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private static JPanel myPanel; private JButton button1; private JButton button2;
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// a panel for contentPane // Define out here to make // visible to ActionListener
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// Define handlers for each event needed (button1, button2) private class Button1Handler implements ActionListener { public void actionPerformed(ActionEvent e) { JOptionPaneshowMessageDialog(frame, "Button 1 pressed"); } } private class Button2Handler implements ActionListener { public void actionPerformed(ActionEvent e) { JOptionPaneshowMessageDialog(frame, "Button 2 pressed"); } } public Simple3() // Construct, build GUI { // Create a panel myPanel = new JPanel(); // Create the buttons button1 = new JButton("Button 1"); button2 = new JButton("Button 2"); // For each component add it ActionListener class button1addActionListener(new Button1Handler()); button2addActionListener(new Button2Handler()); myPaneladd(button1); myPaneladd(button2); } public static void main(String s[]) { Simple3 gui = new Simple3(); // Simple3 component // Adds to current JFrame
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frame = new JFrame("Simple3"); // JFrame for the panel // Standard idiom to catch close event frameaddWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) {Systemexit(0);} }); framegetContentPane()add(myPanel); framepack(); // Ready to go framesetVisible(true); } }
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The sequence diagram in Figure 5-8 now shows that the named listener Button1Handler is used when a user clicks Button1
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Figure 5-8 Sequence diagram for named inner class listener In this example, we are responding to the action event generated when the user clicks on one of the buttons The way these events are handled applies equally well to any Swing event - a menu pick, a list selection, and others Some of these are not ActionEvents with ActionListeners, but include ItemEvents with ItemListeners, and ChangeEvents with ChangeListeners, and so on The general approaches shown in these examples will work for all A Bunch of Options
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On the whole, using the basic features of Swing components is not overly complex But, you won't always want just the basic features Most components have many options that can be used Take a JButton, for example The simplest JButton has a simple text label and takes the default appearance of the GUI look-and-feel But, there are many options You can make it the default button You can change the borders You can change the color You can add an icon to the button You can enable or disable the button You can add a tool tip You can change its size You can change the font And so on This pattern repeats, with some overlap, for almost every Swing control Aside from making it harder to fully understand each component, each one of these options is implemented by a specific method call to the Swing component's class This means that any code that fully takes advantages of a component's options can get long If we set just the options we mentioned for a JButton, it would take at least nine lines of code to define a JButton object Repeat this for each control, and the code for defining a GUI can get long and ugly There are some solutions to this problem The definitions for components can be isolated somewhat from the logic Rather than building a whole interface in the middle of code that has some higher level meaning, you can place the definitions in some method such as createGUI, and then just call it from the higher level code That still means there will be long sequences of repetitious code somewhere If you find you are setting the same component attributes over and over again, you can implement small helper methods that take the attributes as parameters, and then set them all Then the code for defining the component would be a single line call to the helper method, rather than many lines Even so, defining components of the user interface can be complex, and require several lines of code Even though this can make the code longer and more complex, it does keep it all in one place, and portable across the systems Java supports One alternative used by other GUI toolkits is to use resource definition files These resource files have their own set of problems Another alternative is to use software that lets you define GUIs interactively, and then generates all the code automatically Many Java tools will let you do this, in fact But even that approach has some serious problems, which we will discuss in 10
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