What is adapter class? Explain advantages of adapter classes over listener interfaces with suitable example.

 Java Adapter Classes

Java adapter classes provide the default implementation of listener interfaces. If you inherit the adapter class, you will not be forced to provide the implementation of all the methods of listener interfaces. So it saves code.

Pros of using Adapter classes:

• It assists the unrelated classes to work combinedly.
• It provides ways to use classes in different ways.
• It increases the transparency of classes.
• It provides a way to include related patterns in the class.
• It provides a pluggable kit for developing an application.
• It increases the reusability of the class.

The adapter classes are found in java.awt.event, java.awt.dnd and javax.swing.event packages.

 2nd part

Any class which handles events must implement a listener interface for the event type it wishes to handle. A class can implement any number of listener interfaces. To implement a listener interface the class must implement every method in the interface.

Sometimes we may want to implement only one or two of the methods in an interface. We can avoid having to write our own implementation of all the methods in an interface by using an adapter class. An adapter class is a class that already implements all the methods in the corresponding interface. Our class must extend the adapter class by inheritance so we can extend only one adapter class in our class because Java does not support multiple inheritances. Here are the Java adapter classes and the listener interfaces they implement. We can notice that adapters exist for only listener interfaces with more than one method.

Adapter Class

ComponentAdapter

Container Adapter

FocusAdapter

Key Adapter

MouseMotionAdapter

MouseAdapter

Window Adapter

Listener Interface

ComponentListener

ContainerListener

FocusListener

KeyListener

MouseListener

MouseMotionListener

Window Listener