Empty Classes

JMU

Empty Classes
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Interfaces are Very Powerful:
- They give classes the opportunity to "leverage" polymorphism
Realizing Interfaces Can Be Cumbersome:
- Some interfaces have many methods and it can be cumbersome to implement them all

A Workaround

The Idea:
- Always create a class that contains empty implementations of all of the methods and, as needed, create a specialization that overrides some or all of the empty method
The Limitation:
- A derived class can only use this workaround with one interface (since Java uses single inheritance)

An Example

The Interface:
- The MouseListener interface has five methods: mouseClicked(), mouseEntered(), mouseExited(), mousePressed(), and mouseReleased()
A Common Situation:
- Classes only "care about" mouseClicked()

A Design Issue

A Seemingly Obvious Decision:
- Make the empty class abstract to prevent people from misusing it
The Shortcoming of this Decision:
- It prevents you from overriding the empty methods in an anonymous class that is used at construction-time

Empty Classes in Java

An Observation:
- Java has many such classes
A Terminological Issue:
- Java calls them "adapters" but this is confusing because they are not related to the Adapter Pattern (the two were being developed at around the same time and just happened to use the same term)
Some Examples:
- DragSourceAdapter , DropTargetAdapter , KeyAdapter , MouseAdapter , and MouseMotionAdapter

An Example

Using an Anonymous Class