ActionListener

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

class ButtonFrame2 extends JFrame implements ActionListener
{
  JButton bChange;

  ButtonFrame2()     
  {
     . . . . . .
  }

  
  // listener method for the ActionListener interface
  public void actionPerformed( ActionEvent evt)
  {
     . . . . . .
  }

}

A button listener must implement the ActionListener interface. ActionListener is an interface (not a class) that contains a single method:

public void actionPerformed( ActionEvent evt) ;

A class that implements the interface must contain an actionPerformed() method. The ActionEvent parameter is an Event object that represents an event (a button click). It contains information about the event.

Import the package java.awt.event.* if you are dealing with events.

The listener object could be defined in a class other than the frame class. In our example, the class that holds the component is also the listener for its events.