A simple java swing program toggle button with start and stop images in java swing

//A toggle button with start and stop images
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class JTButton extends JFrame implements ActionListener
{
 //vars
 JToggleButton but;
 ImageIcon img1;
  
 JTButton()
 {
  //create content pane with flow layout
  Container c = getContentPane();
  c.setLayout(new FlowLayout());
  
  //image with start signal 
  img1 = new ImageIcon("start.jpg");

  //create toggle button with start image
  but = new JToggleButton("Start/Stop", img1);

  //add button to content pane
  c.add(but);

  //add action listener to button
  but.addActionListener(this);
 }

 public void actionPerformed(ActionEvent ae) 
 {
  //image with stop signal 
  ImageIcon img2 = new ImageIcon("stop.jpg");

  //if toggle button is selected display stop signal image 
  //else display start signal image
  if(but.isSelected())
  but.setIcon(img2);
  else but.setIcon(img1);
   
 }

 public static void main(String args[])
 {
  //create the frame
  JTButton demo = new JTButton();
  demo.setSize(400,400);
  demo.setVisible(true);
  demo.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }
}