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

// This class represents a panel on which 40x40 pixel squares can be drawn.
// The squares are centered around where the user clicks.
public class SquareCanvas extends JPanel implements MouseListener {

    // Keep track of all square center positions
    private ArrayList<Point>  squares;

    // Default constructor
    public SquareCanvas() {
        squares = new ArrayList<Point>();
        setBackground(Color.white);
        addMouseListener(this);
    }

    // This is the method that is responsible for displaying the contents of the canvas
    public void paintComponent(Graphics graphics) {
    	// Draw the component as before (i.e., default look)
        super.paintComponent(graphics);

        // Now draw all of our squares
        graphics.setColor(Color.black);
        for (Point center: squares)
        	graphics.drawRect(center.x-20, center.y-20, 40, 40);
    }

    // These are unused MouseEventHandlers.  Note that we could have 
    // used an Adapter class here.  However, a typical drawing 
    // application would make use of these other events as well.
    public void mouseClicked(MouseEvent event) {}
    public void mouseEntered(MouseEvent event) {}
    public void mouseExited(MouseEvent event) {}
    public void mouseReleased(MouseEvent event) {}

    // Store the mouse location when it is pressed
    public void mousePressed(MouseEvent e) {
        squares.add(e.getPoint());
        repaint();  // this will call paintComponent()
    }

    public static void main(String args[]) {
        JFrame frame = new JFrame("Square Drawing Example");
        frame.add(new SquareCanvas());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 300);
        frame.setVisible(true);
    }
}