Swing repaint() doesn't work in loop or thread

Question

I have the following code:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.text.View;

public class ex10 extends JPanel  {
    private int x=1;
    int y=1;

    //Constructor 
    public ex10() {


        while(true) {

          System.out.println("x ->"+ x);
          System.out.println("y ->" + y);


          x = randomposition(x);
          y = randomposition(y);

          this.repaint();
        }
    }

  public int randomposition(int value) {
        Random random = new Random();

    if (random.nextBoolean() == true) {
      if (value+1 != 500) {
        value++;
      }
    }
    else {
      if (value-1 != 0) {
        value--;  
      }
    }
    return value;
  }
    @Override
    public void paintComponent(Graphics g) {
        //super.paintComponent(g);
        g.setColor(Color.green);
    g.fillRect(x, y, 20, 20);
    }


    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.add(new ex10());
    }

}

Unfortunately, when this.repaint() is called, the point isn't being displayed, but I still got the System.out.println. I tried setting a new thread separatedly, but to no avail. I tried out some other solution (invokelater, and paintimmediately), also to no avail.

My goal is to set a green point which wanders on the screen. Do you have any solution?

Answer 1

Your while (true) is blocking the Swing event thread putting the application to sleep.

For simple animation and game loop, use a Swing Timer. If you have long running code that needs to be in the background, then use a background thread such as a SwingWorker, but taking care to make sure that all calls that change the state of your Swing components should be done on the Swing event thread.

For example, you could change this:

    while(true) {

      System.out.println("x ->"+ x);
      System.out.println("y ->" + y);


      x = randomposition(x);
      y = randomposition(y);

      this.repaint();
    }

to this that uses a Swing Timer (javax.swing.Timer):

int timerDelay = 20;
new Timer(timerDelay, new ActionListener(){
  public void actionPerformed(ActionEvent e) {
    x = randomposition(x);
    y = randomposition(y);
    repaint();
  }
}).start();

---

Regarding DSquare's comments:

• Indeed you are not running your GUI on the Swing event thread, something you should be doing, and yet your while true loop is still freezing your painting because your infinite loop prevents the component from completely creating itself.
• As noted above, you should in fact start all Swing GUI's on the Swing event thread which you could do by placing the Swing creation code into a Runnable and queuing the Runnable on the event thread via the SwingUtilities method, invokeLater.
• You need to call the super's paintComponent method in your paintComponent override so that the JPanel can do its housekeeping graphics works including clearing "dirty" pixels.

---

For example, change this:

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setSize(500, 500);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.add(new ex10());
}

to this:

public static void main(String[] args) {
  SwingUtilities.invokeLater(new Runnable() {
     public void run() {
        JFrame frame = new JFrame();
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.add(new Ex10());
     }
  });
}

And change this:

@Override
public void paintComponent(Graphics g) {
  //super.paintComponent(g);
  g.setColor(Color.green);
  g.fillRect(x, y, 20, 20);
}

to this:

@Override
public void paintComponent(Graphics g) {
  super.paintComponent(g);
  g.setColor(Color.green);
  g.fillRect(x, y, 20, 20);
}