import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JComponent;
import javax.swing.JFrame;
class MyJComponent extends JComponent {
  public void paint(Graphics g) {
    g.setColor(Color.green);
    g.fillRect(30, 30, 100, 100);
  }
}
public class JComponentExample {
  public static void main(String[] arguments) {
    MyJComponent com = new MyJComponent();
    // create a basic JFrame
    JFrame.setDefaultLookAndFeelDecorated(true);
    JFrame frame = new JFrame("JComponent Example");
    frame.setSize(300,200);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    // add the JComponent to main frame
    frame.add(com);
    frame.setVisible(true);
  }
}