Drawing Panel (Vẽ đường thẳng)

package drawingpanel;

/**
 *
 * @author thieumao
 */
import java.awt.*;
import java.util.*;
import java.awt.event.*;
import javax.swing.*;

public class DrawingPanel extends JPanel implements MouseListener {

    int x1, x2, y1, y2;
    boolean draw = false;

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.add(new DrawingPanel());
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public DrawingPanel() {
        super();
        addMouseListener(this);
    }

    public void mouseClicked(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

    public void mouseReleased(MouseEvent e) {
        if (draw) {
            x2 = e.getX();
            y2 = e.getY();
            getGraphics().drawLine(x1, y1, x2, y2);
        } else {
            x1 = e.getX();
            y1 = e.getY();
        }
        draw = !draw;
    }

    public void mousePressed(MouseEvent e) {
    }
}