如何从JPanel触发ActionEvent

Question

I'm trying to repaint a simple massage in a panel by firing an ActionEvent.

I Have a MessagePanel that extends JPanel , in which I defined an addActionListener method and a processEvent method to process the event:

import java.awt.Graphics;
import javax.swing.JPanel; 
import java.util.*;
import java.awt.event.*;

public class MessagePanel extends JPanel {
    private String message = new Date().toString();
    ArrayList<ActionListener> actionListenerList;

    public MessagePanel(String message) {
        this.message = message;
    }

    public void setMessage(String message){
        this.message = message;
    }

    public void addActionListener(ActionListener listener) {
        if (actionListenerList == null) {
            actionListenerList = new ArrayList<>(2);
        }
        if (!actionListenerList.contains(listener)) {
            actionListenerList.add(listener);
        }
    }

    public void removeActionListener(ActionListener listener) {
        if (actionListenerList != null &&
                actionListenerList.contains(listener)) {
            actionListenerList.remove(listener);
        }
    }

    public void processEvent(ActionEvent e) {
        ArrayList<ActionListener> list;

        synchronized(this) {
            if (actionListenerList == null) {
                return;
            }
            list = (ArrayList<ActionListener>)actionListenerList.clone();
        }

        for (int i = 0; i < list.size(); i++) {
            ActionListener listener = (ActionListener)list.get(i);
            listener.actionPerformed(e);
        }     
    }

    @Override
    protected void paintComponent(Graphics g){
         super.paintComponent(g);
         g.drawString(message, 0, 0);
    }
}

Here's my test class:

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

public TestMessaePanel extends JFrame {
    MessagePanel messagePanel = new MessagePanel(new Date().toString());

    public TestMessagePanel() {
        add(messagePanel);
        messagePanel.setCentered(true);

        messagePanel.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e){
                messagePanel.setMessage(new Date().toString());
            }
        });
    }

    public static void main(String[] args) {
        JFrame frame = new TestMessagePanelWithActionEvent();
        frame.setSize(300, 200);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

I'm trying to have the panel repaint the current time with every click of the panel (without using any other event sets) but I can't figure out what event is supposed to fire and invoke my processEvent method. I'm not even really sure if I even need the processEvent , if I can just implement the process elsewhere.

EDIT WITH TEXTBOOK PROBLEM (below)

(Enable MessagePanel to fire ActionEvent) The MessagePanel class in Listing 15.7 is a subclass of JPanel; it can fire a MouseEvent, KeyEvent, and ComponentEvent, but not an ActionEvent. Modify the MessagePanel class so that it can fire an ActionEvent when an instance of the MessagePanel class is clicked. Name the new class MessagePanelWithActionEvent. Test it with a Java applet that displays the current time in a message panel whenever the message panel is clicked, as shown in Figure 36.9.

Answer 1

I'm trying to have the panel repaint the current time with every click of the panel (without using any other event sets)

An ActionListener is to be used only for events that are supposed to trigger it, such as a Timer or an AbstractButton. You should instead use a MouseListener for components that respond to mouse events.

---

Edit Your assignment:

The MessagePanel class in Listing 15.7 is a subclass of JPanel; it can fire a MouseEvent, KeyEvent, and ComponentEvent, but not an ActionEvent. Modify the MessagePanel class so that it can fire an ActionEvent when an instance of the MessagePanel class is clicked. Name the new class MessagePanelWithActionEvent. Test it with a Java applet that displays the current time in a message panel whenever the message panel is clicked, as shown in Figure 36.9.

• You're going to have to give your MessagePanel a MouseListener, one that on mousePressed calls your ActionListener(s).
• In this MouseListener, you're going to have to create an ActionEvent object. Since this is an assignment, I'm not going to show you how to do this but instead will suggest that you go to the ActionEvent API to see what this object needs, and give it try.
• Then you will have to call actionPerformed(...) with the ActionEvent object you've just created on any ActionListeners that need to be called.