Java JButton handle action event

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class Main extends JFrame {
  JTextField field = new JTextField();

  class ButtonListener implements ActionListener {
     boolean toggle = true;

     public void actionPerformed(ActionEvent e) {
       if (toggle)
         field.setText("true");
       else//from  w w  w  .  ja v a  2 s .c o m
         field.setText("false");
       toggle = !toggle;
     }
   }
  public Main() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JButton button = new JButton("Press me");
    button.addActionListener(new ButtonListener());
    getContentPane().add(button, BorderLayout.SOUTH);
    getContentPane().add(field, BorderLayout.NORTH);
    pack();
    setVisible(true);
  }

  public static void main(String[] args) {
     Main st = new Main();
  }

}