Draw Rectangles in Java Swing Example

import javax.swing.*; 
 import java.awt.*; 
 public class JRectanglesJavaExample extends JFrame 
    { 
        Container cntnr = getContentPane(); 
        public JRectanglesJavaExample() 
           { 
               cntnr.setBackground(Color.GREEN); 
               cntnr.setLayout(new FlowLayout()); 
           } 
               public void paint(Graphics gr) 
                 { 
                            super.paint(gr); 
                            gr.setColor(Color.YELLOW); 
                            gr.fillRect(50, 50, 130, 130); 
                            gr.clearRect(60, 70, 60, 60); 
                } 
                           public static void main(String[] ag) 
                            { 
                                 JRectanglesJavaExample frm = new JRectanglesJavaExample(); 
                                 frm.setSize(230,230); 
                                 frm.setVisible(true); 
                            } 
    }