import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;

public class ClickMe extends Applet implements MouseListener {
    private Spot spot = null;
    private static final int RADIUS = 7;

    public void init() {
	addMouseListener(this);
    }

    public void paint(Graphics g) {
	//draw a black border and a white background
        g.setColor(Color.white);
	g.fillRect(0, 0, getSize().width - 1, getSize().height - 1);
        g.setColor(Color.black);
	g.drawRect(0, 0, getSize().width - 1, getSize().height - 1);

	//draw the spot
        g.setColor(Color.red);
	if (spot != null) {
	    g.fillOval(spot.x - RADIUS, spot.y - RADIUS, RADIUS * 2, RADIUS * 2);
        }
    }
    public void mousePressed(MouseEvent event) {	
        if (spot == null) {
            spot = new Spot(RADIUS);
        }
	spot.x = event.getX();
	spot.y = event.getY();
	repaint();
    }
    public void mouseClicked(MouseEvent event) {}
    public void mouseReleased(MouseEvent event) {}
    public void mouseEntered(MouseEvent event) {}
    public void mouseExited(MouseEvent event) {}
}

public class Spot {
    public int size;
    public int x, y;

    public Spot(int intSize) {
        size = intSize;
        x = -1;
        y = -1;
    }
}

<applet code="ClickMe.class"
        width="300" height="150">
</applet>