import java.awt.Color;               import java.awt.Container;
import java.awt.Dimension;           import java.awt.Graphics;
import java.awt.Point;               import java.awt.event.MouseEvent;
import java.awt.event.MouseListener; import java.util.ArrayList;
import javax.swing.JComponent;       import javax.swing.JFrame;

public class ClickComponent extends JComponent implements MouseListener {
    private ArrayList<Point> centers;

    public ClickComponent() {
        centers = new ArrayList<Point>();
        addMouseListener(this);
        setPreferredSize(new Dimension(250, 250));
    }

    public void paintComponent(Graphics g) {
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(new Color(0, 128, 0));
        for(int i = 0; i < centers.size(); i++) {
            Point p = centers.get(i);
            int px = (int) Math.round(p.getX());
            int py = (int) Math.round(p.getY());
            g.fillOval(px - 10, py - 10, 20, 20);
        }
    }

    public void mousePressed(MouseEvent e) {
        centers.add(new Point(e.getX(), e.getY()));
        repaint();
    }
    public void mouseReleased(MouseEvent e) { }
    public void mouseEntered(MouseEvent e) { }
    public void mouseExited(MouseEvent e) { }
    public void mouseClicked(MouseEvent e) { }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Click Component");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Container contents = frame.getContentPane();
        contents.add(new ClickComponent());
        frame.pack();
        frame.setVisible(true);
    }
}