event source => listener

package HeadFirstJava;

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

import javax.swing.*;

public class SimpleGui {
JButton button;

public static void main(String[] args) {
SimpleGui gui = new SimpleGui();
gui.go();
}
public void go() {
JFrame frame = new JFrame();
button = new JButton("click me");
// event registration
// button is 'event source'
button.addActionListener(new ButtonListener());
frame.getContentPane().add(BorderLayout.EAST, button);
frame.setSize(300, 300);
frame.setVisible(true);
}
// listener class
class ButtonListener implements ActionListener {

@Override
public void actionPerformed(ActionEvent event) {
// TODO Auto-generated method stub
button.setText("Ouch");
}

}
}