Complete Program

The following is a complete program, suitable for copying to a file and running.

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

class ButtonFrame2 extends JFrame implements ActionListener
{
  JButton bChange ; // reference to the button object

  // constructor for ButtonFrame2
  ButtonFrame2(String title) 
  {
    super( title );                   // invoke the JFrame constructor
    setLayout( new FlowLayout() );    // set the layout manager

    // construct a Button
    bChange = new JButton("Click Me!"); 
    
    // register the ButtonFrame2 object as the listener for the JButton.
    bChange.addActionListener( this ); 

    add( bChange );                   // add the button to the JFrame
    setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );   
  }
  
  public void actionPerformed( ActionEvent evt)
  {
    getContentPane().setBackground( Color.blue );     // Change the Frame's background
    repaint();  // ask the system to paint the screen.
  }
}

public class ButtonDemo2
{
  public static void main ( String[] args )
  {
    ButtonFrame2 frm = new ButtonFrame2("Button Demo Two");

    frm.setSize( 200, 100 );     
    frm.setVisible( true );   
  }
}

The repaint() method called in actionPerformed()tells the system to repaint the screen sometime soon because we have changed something. The system will do this when it is ready. If you don't call repaint(), the frame might not change color until you do something that would ordinarily cause the frame to be painted such as resizing or moving the frame.