Program with a JPanel

Here is a program that creates a JFrame, creates a JPanel, and adds the panel to the frame. Everytime the frame is painted a diagonal line is drawn on the panel. The frame is painted whenever the monitor screen needs to be updated, for instance, when the frame is moved or resized with the mouse.

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

class DemoPanel extends JPanel
{

  public DemoPanel()
  {
    setPreferredSize( new Dimension(400, 300) );
    setBackground( Color.WHITE );    
  }  
 
  public void paintComponent ( Graphics gr )
  { 
    int width  = getWidth();
    int height = getHeight();
    
    super.paintComponent( gr );
    gr.setColor( Color.BLUE );
    gr.drawLine( 0, 0 , width-1, height-1 );
  }
}

public class TestPanel
{
   public static void main ( String[] args )
   {
      JFrame frame = new JFrame( "Test Panel" );
      frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      
      frame.getContentPane().add( new DemoPanel() );
      
      frame.pack();
      frame.setVisible( true );
   }
}

This line from main():

frame.getContentPane().add( new DemoPanel() );