Where is the actionperfomed() method called?

Question

I'm a Java beginner and now as I come down to work with interfaces, I want to know what really happens. I suppose a good example is the ActionListener interface.

All I know about interfaces is, that it forces you to implement the methods given by the interface. But what I don't get, where the actionPerformed(ActionEvent e) method is called. And are there any simple examples to show me what happens in the background? Thanks anyway.

Answer 1

If you want details, log an Exception from inside ActionListener#actionPerformed :

import java.io.*;
import javax.swing.*;
import java.awt.event.*;

public class ListenerTracer extends JPanel {

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() { createAndShowGUI(); }
        });
    }

    public ListenerTracer() {
        JButton b1 = new JButton("Press me");
        b1.setVerticalTextPosition(AbstractButton.CENTER);
        b1.setHorizontalTextPosition(AbstractButton.CENTER);
        b1.addActionListener(new ActionListener() {
            @Override public void actionPerformed(ActionEvent event) {
                Exception e = new Exception();
                e.printStackTrace();
            }
        });
        add(b1);

        JTextArea textArea = new JTextArea("actionListener printstacktrace:\n", 50, 50);
        JScrollPane scrollPane = new JScrollPane(textArea);
        add(scrollPane);
        Console.redirectOutput(textArea);
    }

    private static void createAndShowGUI() {
        JFrame frame = new JFrame("ListenerTracer");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ListenerTracer contentPane = new ListenerTracer();
        contentPane.setOpaque(true);
        frame.setContentPane(contentPane);
        frame.pack();
        frame.setVisible(true);
    }

    private static class Console implements Runnable {
        JTextArea displayPane;
        BufferedReader reader;

        private Console(JTextArea displayPane, PipedOutputStream pos) {
            this.displayPane = displayPane;
            try {
                PipedInputStream pis = new PipedInputStream(pos);
                reader = new BufferedReader( new InputStreamReader(pis) );
            }
            catch (IOException e) {}
        }

        public void run() {
            String line = null;
            try {
                while ((line = reader.readLine()) != null) {
                    displayPane.append( line + "\n" );
                    displayPane.setCaretPosition(displayPane.getDocument().getLength());
                }
                System.err.println("im here");
            }
            catch (IOException ioe) {
                JOptionPane.showMessageDialog(null, "Error redirecting output : "+ioe.getMessage());
            }
        }

        public static void redirectOutput(JTextArea displayPane) {
            PipedOutputStream pos = new PipedOutputStream();
            System.setErr(new PrintStream(pos, true) );
            Console console = new Console(displayPane, pos);
            new Thread(console).start();
        }
    }
}

Clicking the "Press Me" button produces this output:

actionListener printstacktrace: java.lang.Exception at ListenerTracer$2.actionPerformed(ListenerTracer.java:21) at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.setPressed(Unknown Source) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) at java.awt.Component.processMouseEvent(Unknown Source) at javax.swing.JComponent.processMouseEvent(Unknown Source) at java.awt.Component.processEvent(Unknown Source) at java.awt.Container.processEvent(Unknown Source) at java.awt.Component.dispatchEventImpl(Unknown Source) at java.awt.Container.dispatchEventImpl(Unknown Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) at java.awt.LightweightDis patcher.dispatchEvent(Unknown Source) at java.awt.Container.dispatchEventImpl(Unknown Source) at java.awt.Window.dispatchEventImpl(Unknown Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.EventQueue.dispatchEventImpl(Unknown Source) at java.awt.EventQueue.access$400(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue$4.run(Unknown Source) at java.awt.EventQueue$4.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknown Source) at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) at java.awt.EventDispatchThread.pumpEv entsForFilter(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.run(Unknown Source)

I borrowed the console redirect code from camickr's answer to redirecting-system-out-to-jtextpane .

Answer 2

That will be called when the user does something. Click a button, select a menu. Each one of the actionListener s is called for a particular event. I think right now, you need not worry exactly where it is called, just where, and why.

Answer 3

In the case of a JButton, its superclass AbstractButton calls the actionPerformed method through the method fireActionPerformed which is a protected method. If you check the source code you'll see that it constructs an ActionEvent object and calls actionPerformed using it as an argument.