Simple animation won't work

[nolsen01]

I'm following a tutorial. The tutorial is using AWT but I chose to use swing instead so that may have something to do with it. Its very simple code so I'll post it here.

The idea is to just move a 2D ball from left to right across the screen. However, instead of repainting the screen, it drags the ball across the screen and leaves a thick red line where the ball was moving.

package org.me.hello;

/**
 *
 * @author Nick
 */
import java.awt.Color;
import javax.swing.*;
import java.awt.Graphics;


public class MyApplet extends JApplet implements Runnable{

    private int x_pos = 10;
    private int y_pos = 100;
    private int radius = 20;

    
    @Override
    public void init(){
        setBackground(Color.blue);
    }

    @Override
    public void start(){
        //Thread.start runs the run() method of the target class
        //in this case, "this"
        Thread th = new Thread(this);
        th.start();
    }

    @Override
    public void stop(){

    }

    @Override
    public void destroy(){

    }

    @Override
    public void paint(Graphics g){
        g.setColor(Color.red);
        g.fillOval(x_pos - radius, y_pos - radius, 2 * radius, 2 * radius);
    }




    public void run() {

        //set currentThread to lowest priority
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);

        while(true){
            
            x_pos++;

            repaint();

            //make this thread sleep for 20 milliseconds
            try{
                Thread.sleep(20);
            }catch(InterruptedException ex){
                //do nothing
            }
            
            Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
        }
    }




}

[Fubarable]

To fully repaint the screen you need to have the super paint method called as the first method of the paint method. In other words, instead of this:

    public void paint(Graphics g){
        g.setColor(Color.red);
        g.fillOval(x_pos - radius, y_pos - radius, 2 * radius, 2 * radius);
    }

use this:

    public void paint(Graphics g){
        super.paint(g); // repaints the background of the component
        g.setColor(Color.red);
        g.fillOval(x_pos - radius, y_pos - radius, 2 * radius, 2 * radius);
    }

[Fubarable]

Having said this, if you really want to do Swing animation, you'll make other changes:

1) Draw in a JPanel, not directly in the JApplet itself.
2) Override the JPanel's paintComponent method, not the paint method.
3) Use a Swing Timer instead of a while (true) loop.
4) Call the Swing code using SwingUtilities invoke and wait for thread safety.

For instance:

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

import javax.swing.*;

public class SwingAnimation2 extends JApplet {
  @Override
  public void init() {
    try {
      // start Swing in a thread-safe way
      SwingUtilities.invokeAndWait(new Runnable() {
        public void run() {
          // add an animation panel object to the applet's contentPane
          getContentPane().add(new AnimationPanel());
        }
      });
    } 
    // to the purist's out there, sorry for catching a naked Exception... 
    catch (Exception e) {
      System.err.println("Applet failed to initialize");
    }
  }
}

class AnimationPanel extends JPanel {
  private static final int RADIUS = 20;
  private static final int TIMER_DELAY = 20;
  private int  xPos = 10;
  private int yPos = 100;

  public AnimationPanel() {
    setBackground(Color.blue);
    javax.swing.Timer timer = new javax.swing.Timer(TIMER_DELAY, new ActionListener() {
      public void actionPerformed(ActionEvent arg0) {
        xPos++;
        AnimationPanel.this.repaint();
      }
    });
    timer.start();
  }
  
  @Override
  // override a Swing JComponent's paintComponent, not the paint method
  protected void paintComponent(Graphics g) {
    super.paintComponent(g); // again to repaint the screen
    
    // set antialiasing rendering hints to smooth out the circle
    // this is not necessary but makes for prettier animation
    Graphics2D g2 = (Graphics2D)g;
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
        RenderingHints.VALUE_ANTIALIAS_ON);
    
    g.setColor(Color.red);
    g.fillOval(xPos  - RADIUS, yPos  - RADIUS, 2 * RADIUS, 2 * RADIUS);
  }
}

[nolsen01]

oh i see. I guess because JApplet overloads the paint method from the original Applet class?

Thank you.

[Fubarable]

You really should not draw directly in a JApplet or JFrame. Much better is to draw in a JPanel or JComponent (see my example code above). The Sun Swing tutorials are very helpful here and it would likely help you a great deal if you had a look at the Swing graphics and 2D graphics sections.