GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 350

Notice the following:
￿ The class CalculateButtonHandler starts with the modifier
private . This is because you want this class to be used only within
your RectangleProgram .
￿ This class uses another modifier, implements . This is how you build
classes on top of classes that are interfaces. Notice that you have not yet
provided the code for the method actionPerformed . You will do that
shortly.
In Java, implements is a reserved word.
Next, we illustrate how to create a listener object of type CalculateButtonHandler .
Consider the following statements:
CalculateButtonHandler cbHandler;

cbHandler = new CalculateButtonHandler(); //instantiate the object

As described, these statements create the listener object. Having created a listener, you
next must associate (or in Java terminology, register) this handler with the corresponding
JButton . The following line of code registers cbHandler as the listener object of
calculateB :

calculateB.addActionListener(cbHandler);

The complete definition of the class CalculateButtonHandler , including the code
for the method actionPerformed , is:

private class CalculateButtonHandler implements
ActionListener //Line 1

{

public void actionPerformed(ActionEvent e) //Line 2

{

double width, length, area, perimeter; //Line 3

length
= Double.parseDouble(lengthTF.getText());

//Line 4

width
= Double.parseDouble(widthTF.getText()); //Line 5

area = length * width; //Line 6

perimeter = 2 * (length + width); //Line 7

areaTF.setText("" + area); //Line 8

perimeterTF.setText("" + perimeter); //Line 9

}

}
In the preceding program segment, Line 1 declares the class
CalculateButtonHandler and makes it an action listener by including the phrase
implements ActionListener . Note that all of this code is just a new class definition.