Re: Questions about drawing on Canvas

Knute Johnson <>
Wed, 30 Apr 2008 14:17:09 -0700
Zerex71 wrote:

I finally got some things to draw today, and realized that I never
needed Canvas in the first place, (a) because there was no canvas in
the two examples I was looking at and (b) I overlooked the fact that
Canvas is AWT and not Swing. I'm still getting reacclimated to the
AWT-vs-Swing paradigm.

My new problem is probably somewhat simpler: I am drawing a set of
visual axes (VisualAxes being an extension of JPanel) and I also want
to add another vector/line to the drawing area (VisualVector which is
an extension of JPanel). I have code to new the VisualAxes and add
them to the frame, then new the VisualVector and add it to the frame.
The result however is that I cover up the axes with the subsequently-
added VisualVector. (Incidentally, VisualAxes is composed of three
VisualVectors, which I am finally able to draw appropriately on the
screen at a desired origin.)

For the snippet folks:

        Vector origin = new Vector(350.0, 350.0, 0.0);
        double axisLength = 50.0; // Set a reasonable length for each
        VisualAxes axes = new VisualAxes(fg, origin, axisLength);
        VisualVector vv = new VisualVector(new Vector(100.0, 200.0,
50.0),, origin);
        vv.setOpaque(false); // See if this allows us to not cover up
the axes
        frame.pack(); // ALWAYS the second-to-last thing to do
        frame.setVisible(true); // ALWAYS the last thing to do



You are adding two components to the JFrame, a VisualVector and
VisualAxes, without specifying any constraints. I don't know what
layout manager you are using but depending on which one it could cause
both of those components to occupy the same space on the JFrame and make
that first one not visible.

I have a suggestion and that is to use a JPanel as your drawing surface
and draw your axes and vectors directly onto it. Don't create
components for your drawing elements.

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

public class test extends JPanel implements ActionListener {
     int shape = 0;

     public test() {
         setPreferredSize(new Dimension(400,300));

     public void actionPerformed(ActionEvent ae) {
         String ac = ae.getActionCommand();
         if (ac.equals("Circle")) {
             shape = 1;
         } else if (ac.equals("Square")) {
             shape = 2;

     public void paintComponent(Graphics g) {
         // fill background with gray

         // draw xy axis

         // draw the appropriate shape
         switch (shape) {
             case 1: g.drawOval(10,10,280,280);
             case 2: g.drawRect(10,190,100,100);

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 JFrame f = new JFrame("test");
                 final test t = new test();

                 JPanel p = new JPanel(new FlowLayout());
                 JButton b = new JButton("Circle");

                 b = new JButton("Square");


Again I suggest that you look at the code for my Asteroids game to give
you an idea how to do drawing and animation.


Knute Johnson
email s/nospam/linux/

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