为什么当同一类的对象的方法调用repaint（）时不起作用？

Question

In a program I'm building for my class I have the same class extending a Swing JPanel and implementing MouseListener, for which I use two instantiations - one to function as a JPanel, and the other as a mouse listener for that JPanel.

But when I click in the window, repaint() the MouseClicked method in the mouse listener fails to call the first object's paintComponent() method. For example:

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

public class TestPanel extends JPanel implements MouseListener{
    static boolean black;
    static TestPanel test = new TestPanel();

    public void mouseExited(MouseEvent e){}
    public void mousePressed(MouseEvent e){}
    public void mouseReleased(MouseEvent e){}
    public void mouseEntered(MouseEvent e){}
    public void mouseClicked(MouseEvent e){ //Expected behavior: the square turns black immediately
        System.out.println("CLICK!");
        black = true;
        test.repaint(); //this fails
        try{
            Thread.sleep(3000);
        }catch(Exception ex){}
    }

    public void paintComponent(Graphics g){
         super.paintComponent(g);
         Graphics2D g2d = (Graphics2D) g;
         System.out.println("Painting...");
         g2d.setColor(Color.white);
         if(black){
             g2d.setColor(Color.black);
         }
         g2d.fillRect(0, 0, 200, 200);
    }

    public static void main(String[] args) throws InterruptedException{
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        test.addMouseListener(new TestPanel());
        test.setPreferredSize(new Dimension(200, 200));
        frame.add(test);
        frame.pack();
        frame.setVisible(true);
        while (true){
            black = false;
            test.repaint();
            Thread.sleep(100);
        }
    }
}

If you watch what happens on a click, the screen stays white for the 3 seconds after the click is registered, until the loop starts up again, ie, the repaint() call in the mouse listener didn't work. Why does this happen?

I'm guessing it would work if I made different classes for the objects, but I'm mostly curious as to why it doesn't work this way.

Answer 1

for which I use two instantiations - one to function as a JPanel, and the other as a mouse listener for that JPanel.

There is no need to do that. All you need is a single instance of the TestPanel class.

In the constructor of your TestPanel class you just add:

addMouseListener( this);

The get rid of the static variable for the TestPanel class.

Then the code in your main method should look something like:

    //test.addMouseListener(new TestPanel());
    //test.setPreferredSize(new Dimension(200, 200));
    //frame.add(test);
    frame.add( new TestPanel() );  

Also, the TestPanel class should override the getPreferredSize() method to return the Dimension of your panel.

Read the section from the Swing tutorial on Custom Painting for a working example with a MouseListener .

Answer 2

The AWT thread is responsible for calling MouseListener and for repaint. Inside the repaint(); method, the AWT thread is told to call the paint(); Just call it using a different thread. In general, it is a bad idea to do anything intensive with the AWT thread. It already does a lot, taking too much of its time will mess your GUI up.

Depending on your needs, this might work:

new Thread(()->{repaint();}).start();