Not Quite Correct Program

Here is the program with an actionPerformed() method added in the correct place. You can copy this program to a file, compile, and run it. However, there is something wrong with it.

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

public class TwoButtons extends JFrame implements ActionListener
{
  JButton redButton ;
  JButton grnButton ;

  // constructor for TwoButtons
  public TwoButtons()                           
  {
    redButton = new JButton("Red");
    grnButton = new JButton("Green");
    getContentPane().setLayout( new FlowLayout() ); 
    getContentPane().add( redButton );                      
    getContentPane().add( grnButton );
    

    // register the buttonDemo frame
    // (the frame this constructor is making)
    // as the listener for both Buttons.
     
    redButton.addActionListener( this );
    grnButton.addActionListener( this );      
    
    setDefaultCloseOperation( WindowConstants.EXIT_ON_CLOSE );   
  }

  public void actionPerformed( ActionEvent evt)
  {
    getContentPane().setBackground( Color.blue );
    repaint();
  }
  
  public static void main ( String[] args )
  {
    TwoButtons demo  = new TwoButtons() ;
    
    demo.setSize( 200, 150 );     
    demo.setVisible( true );      
  }
}

Check back to the description of what this program is supposed to do (its specifications) and determinine where the current version of the program errors.