Re: How do I paint on an existing Panel?

Knute Johnson <>
Wed, 18 Jan 2012 15:00:10 -0800
On 1/18/2012 10:12 AM, A B wrote:

"Knute Johnson" <> wrote in message

On 1/18/2012 8:55 AM, A B wrote:

I expect this is a really obvious question; I must get round to getting
a bigger Java book! Part of my program starts by displaying a JPanel,
with an image put on it using paintComponent(). How do I then draw some
shapes over the top of that image when a particular button is clicked? I
can't just put them in paintComponent(), because they haven't to appear
at first. Would it be easier in an applet, rather than an application?

I'm sure this comes up in the example programs in the JDK package, but
for some mysterious reason they won't work. Wrong version of IE,
probably; have to look into that.

Put all your drawing code in the paintComponent() method. One idea,
you could stash your shapes in a List and in the paintComponent()
method, retrieve and paint them. Press a button put a shape in the
List, call repaint().

I'm not quite sure what you mean. If the drawing code was in
paintComponent(), what's to stop it executing when the JPanel first
appears? And what do Lists do, they aren't in my book? Sorry to be

I had to look up gormless. You must be in the UK? You need to read the
docs for java.util.List. To answer your question, you don't paint it
unless it needs to be painted. There are numerous ways to do that, with
just two examples in the code below.

You could also look at the code for my Asteroids game, Lots of shapes in there.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.image.*;
import java.util.*;
import javax.imageio.*;
import javax.swing.*;

public class test extends JPanel implements ActionListener {
     private final Random random =
      new Random(System.currentTimeMillis());
     private BufferedImage image;
     private final java.util.List<ColoredShape> shapeList =
      new java.util.ArrayList<ColoredShape>();

     public test() {
         try {
             URL url =
              new URL("");
             image =;
             setPreferredSize(new Dimension(
         } catch (MalformedURLException murle) {
              // you need to catch these
         } catch (IOException ioe) {

     public void actionPerformed(ActionEvent ae) {
         double w = random.nextDouble() * getWidth();
         double h = random.nextDouble() * getHeight();
         double x = random.nextDouble() * getWidth() -
          0.5 * getWidth();
         double y = random.nextDouble() * getHeight() -
          0.5 * getHeight();
         Color color = new Color(
         ColoredShape shape = new ColoredShape(x,y,w,h,color);

     // do all of your drawing in the overridden method paintComponent()
     public void paintComponent(Graphics g2d) {
         Graphics2D g = (Graphics2D)g2d;

         if (image != null)

         for (ColoredShape shape : shapeList) {

     class ColoredShape extends Ellipse2D.Double {
         private final Color color;

         public ColoredShape(double x,double y,double w,double h,
          Color color) {
             this.color = color;

         public Color getColor() {
             return color;

     public static void main(String[] args) {
         // create GUI on EDT (event dispatch thread)
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 JFrame f = new JFrame();
                 test t = new test();
                 JButton b = new JButton("Add Shape");


Knute Johnson

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