我可以在java中使用addActionListener（this）代替“ this”吗？

Question

Say, I have a class XFrame that extends JFrame and implements ActionListener. In the class I have a JButton b and I overrode the actionPerformed method. Now in the constructor I have to set b.addActionListener(this). My question is: what does 'this' do in this case and what can I replace that with? I tried b.addActionListener(new XFrame()) but it didn't work.

Answer 1

Try an anonymous class:

public class Test {

    public static void main(final String[] args) {
        final JFrame frame = new JFrame();
        final JButton button = new JButton("Test");
        button.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(final ActionEvent e) {
                System.out.println("pressed");
            }
        });

        frame.getContentPane().add(button);
        frame.pack();
        frame.setVisible(true);
    }
}

Or if you want to make your JFrame implement ActionListener instead of using an anonymous class, see this example . In this case, this would refer to the instance of the JFrame .

Answer 2

What do you mean by "it didn't work". I expect that it compile fine but it did not what you expected. It is normal because you are saying to swing that the actionListener is not the one from the XFrame that received the event, but the actionlistener related to another XFrame (the new XFrame() as parameter). This another XFrame may not even be displayed on the screen ! So don't expect to see anything ;)

To answer your question, you can replace "this" by any instance of ActionListener. Your XFrame happens to be one hence the "this". But you could replace it by any other actionListener created anywhere else. The actionListener WILL be notified. But the important point is that this ActionListener has to do something interesting (visible!) upon notification. And a "new XFrame()" can not as it is not displayed on screen ! Only the currently displayed on screen XFrame can.

In a complex system, you generally do not want your Frame to be also the one handling events because you want to separate the concepts. But in the simple programs you do when learning Java, it is usually easier to visually see a feedback given by the displayed Frame upon clicking a button.

Answer 3

what does 'this' do in this case ...?

The this passed to b.addActionListener(this) tells the button it should call the actionPerformed method of this object (ie that is the XFrame instance) whenever the action of the button is triggered (ie the button is clicked).

what can I replace that with?

You can replace it with any other object that implements the ActionListener interface. Often this is done through an 'anonymous' class, as for the otherActionListener in the example below.

Another nice thing: you not only can 'replace' the this with some other ActionListener object, you may even add multiple ActionListener objects to the button. Eg in the example below I added the XFrame instance ( this ) and the otherActionListener . When the button is clicked the actionPerformed methods of both objects will be executed.

Here a complete example:

import static javax.swing.JOptionPane.showMessageDialog;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;

public class XFrame extends JFrame implements ActionListener {
    JButton b = new JButton("Click me");

    ActionListener otherActionListener = new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            showMessageDialog(XFrame.this, "The other action says: 'Hi'");
        }
    };

    public XFrame() {
        getContentPane().add(b);

        b.addActionListener(this);

        b.addActionListener(otherActionListener);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
       showMessageDialog(this, "XFrame says: 'hello'");
    }

    public static void main(String[] args) {
        XFrame frame = new XFrame();
        frame.setSize(100,60);
        frame.setVisible(true);
    }
}

Answer 4

Take a look at this (pun intended) article. Also, another way you can do it is this way:

ActionListener e = new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent arg0) {
            // TODO Auto-generated method stub
        }
};
b.add(e);