GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 349

￿ For each JButton , you must specify the corresponding listener object. In
Java, this is known as registering the listener.
￿ You must define the methods that will be invoked when the event is sent
to the listener. Normally, you will write these methods and you will
never write the code for invocation.
Java provides various classes to handle different kinds of events. The action event is
handled by the class ActionListener , which contains only the method
actionPerformed . In the method actionPerformed , you include the code that you
want the system to execute when an action event is generated.
The class ActionListener that handles the action event is a special type of class,
called an interface . In Java, interface is a reserved word. Roughly speaking, an
interface is a class that contains only the method headings, and each method

heading is terminated with a semicolon. For example, the definition of the

interface ActionListener containing the method actionPerformed is:

6

public interface ActionListener
{

public void actionPerformed(ActionEvent e);

}

Because the method actionPerformed does not contain a body, Java does not allow
you to instantiate an object of type ActionListener . So how do you register an action
listener with the object calculateB ?
One way is as follows (there are other ways not discussed here): Because you cannot
instantiate an object of type ActionListener , first you need to create a class on top of
ActionListener so that the required object can be instantiated. The class created must
provide the necessary code for the method actionPerformed . You will create the
class CalculateButtonHandler to handle the event generated by clicking the button
calculateB .
The class CalculateButtonHandler is created on top of the interface
ActionListener . The definition of the class CalculateButtonHandler is:
private class CalculateButtonHandler implements
ActionListener //Line 1

{

public void actionPerformed(ActionEvent e) //Line 2

{

//The code for calculating the area and the perimeter
//and displaying these quantities goes here

}

}