将自定义JPanel添加到JFrame之后的repaint方法

Question

I have this code, but JPanel repaint method does not work. If I do that after add it to JFrame class I debug it and it does not recall paintComponent .

Why is that so?

package trial;

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;


class jp extends JPanel
{
     int i=0;

     {
        setBackground(Color.RED);
     }

     @Override
     protected void paintComponent(Graphics g)
     { 
        super.paintComponent(g);
        Color[] c={Color.red,Color.green};
        //To change body of generated methods, choose Tools | Templates.
        g.drawLine(i, i, i+100, i+100);
        i+=50;
     }
}

public class Trial extends JFrame{

   public static void main(String[] args) {
       Trial f=new Trial();
       jp a;
       a=new jp(); 
       System.out.println(a.i);
       f.add(a);
       f.setVisible(true);
       f.setSize(500, 500);
       f.setDefaultCloseOperation(3);

       a.repaint();

   }
}

Answer 1

[UPDATE] to fit the question better: A call to repaint() places a PaintEvent into the Event Queue . This PaintEvent will be handled by another thread, the Event Dispatching Thread .

In short: the paintComponent() is called in another thread as your main method.

[/UPDATE]

I am not sure what you're trying to achive but try this at the end of your main method:

    try {
        while (true) {
            Thread.sleep(1000);
            a.repaint();
        }
    } catch (InterruptedException e) {
    }

Is a moving line like this what you're expecting? The reason why this is working: As far as I recall, Java Swing may collapse multiple repaint requests that are enqueued in the Event Dispatching Thread into a single paint request - which is probably what you experienced. In my example above, waiting 1 second is enough to get distinct repaints.

Nevertheless I strongly advise you not to use my (or your) code . Reason: The value of i depends on the number of calls to paintComponent - which can vary. For example, pull at a corner to resize the window - repaint maybe gets called dozens of times then!

Perhaps you want a javax.swing.Timer to periodically update the value of i and then trigger a repaint?