keeping shapes on a canvas with a paint tool

oops!

public void paintComponent(Graphics g){
super.paintComponent(g);
....}

First, comment out the line
super.paint(g)

Second, the update method of your canvas: update(g)
should be overridden to only call the paint method:

public void update(Graphics g){
paint(g);
}

That'll fix your problem of having the shapes disappear w/ each successive click by the user.
I can't exactly explain the logic of how all this works but if you try out what I say it will indeed work.
Perhaps another forum user will be able to better explain how the painting works.

Sorry guys, but that doesn't fix the problem, if I minimize/restore this frame
then only the last shape is visible. I call this method "drawing in the sand".

But, we just fixed that in another thread:

http://forums.dev-archive.com/showthread.php?p=440144#post440144

Here I have "fixed" your code using double buffering, and the method could
well be called "tattoing" :) . Its not a good one, the link above shows the
preferred way of solving this.

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;

public class DTF
extends Frame
implements WindowListener {
DrawToolCanvas myCanvas; // member variable

public DTF() {
setTitle("Draw Tool Frame");
addWindowListener(this);
Button square, circle;
Panel myPanel = new Panel();
square = new Button("square");
square.setActionCommand("square");
circle = new Button("circle");
circle.setActionCommand("circle");
myPanel.add(square);
myPanel.add(circle);
add("South", myPanel);
myCanvas = new DrawToolCanvas(); // member variable
add("Center", myCanvas);
square.addActionListener(myCanvas);
circle.addActionListener(myCanvas);
setSize(400, 400);
setLocation(200, 200);
setVisible(true);
myCanvas.prepBuffer();
}

public void windowClosing(WindowEvent event) {
System.exit(0);
}

public void windowOpened(WindowEvent event) {}

public void windowIconified(WindowEvent event) {}

public void windowDeiconified(WindowEvent event) {}

public void windowClosed(WindowEvent event) {}

public void windowActivated(WindowEvent event) {}

public void windowDeactivated(WindowEvent event) {}

public static void main(String[] args) {
new DTF();
}
}

class DrawToolCanvas
extends Canvas
implements MouseListener, ActionListener {
int x, y;
boolean drawCircle;
BufferedImage memImg=null;
Graphics memG=null;

public DrawToolCanvas() {
x = -40;
y = -40;
drawCircle = true;
addMouseListener(this);
prepBuffer();
}

public void prepBuffer() {
memImg=new BufferedImage(1024,1024,BufferedImage.TYPE_INT_RGB);
memG=memImg.getGraphics();
memG.setColor(Color.white);
memG.fillRect(0,0,getWidth(), getHeight());
}
public void update(Graphics g) {
paint(g);
}
public void paint(Graphics g) {
if (drawCircle) {
memG.setColor(Color.blue);
memG.fillOval(x, y, 40, 40);
}
else {
memG.setColor(Color.red);
memG.drawRect(x, y, 40, 40);
}
g.drawImage(memImg,0,0,this);
}

public void actionPerformed(ActionEvent event) {
if (event.getActionCommand().equals("square"))
drawCircle = false;
else if (event.getActionCommand().equals("circle"))
drawCircle = true;
}

public void mousePressed(MouseEvent e) {
x = e.getX();
y = e.getY();
repaint();
}

public void mouseEntered(MouseEvent e) {}

public void mouseExited(MouseEvent e) {}

public void mouseReleased(MouseEvent e) {}

public void mouseClicked(MouseEvent e) {}
}