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Introductory discussions of the difference between interfaces and abstract classes exemplify the implementation-centric view adopted by most Java texts. Such discussions explain how to use either an interface or an abstract class, but seldom explain why you would choose to use one or the other. Through a simple example, this article investigates the design decisions driving the use of interfaces and abstract classes and shows why interfaces satisfy type concerns and abstract classes satisfy implementation concerns.
To lay the groundwork necessary for a clear discussion, I review Java class and interface constructs from both type and implementation-centric viewpoints. As detailed in my earlier article, "Thanks Type and Gentle Class" (JavaWorld, January 2001), classes and interfaces establish a system's type hierarchy, whereas only classes establish the implementation hierarchy. Classes divide into two types: concrete and abstract.
I'll briefly review three constructs: concrete classes, abstract classes, and interfaces. I start with the concrete class.
In Java, because concrete classes do not require a special designation during declaration, they are typically just called classes. Each declared class performs double duty. From a type perspective, the class defines a type and a set of operations for the class's object instances. From an implementation perspective, the class provides implementation code for each declared operation. These implementation modules are the class methods.
Classes inherit operations from supertypes and methods from superclasses. The restriction that each class extend only one direct superclass means the implementation hierarchy follows a single inheritance policy. Since a class can implement one or more interfaces and extend one other class, type operations can be inherited from multiple supertypes. Thus, Java supports multiple inheritance in the type hierarchy.
| Concrete class | |
|---|---|
| Type-centric | Implementation-centric |
| Defines a type and a set of operations on that type | Provides implementation for each declared type operation |
The abstract keyword in the class declaration identifies an abstract class. From a type perspective, the type-defining characteristics
of an abstract class match those of a concrete class. The two differ in that an abstract class optionally provides implementation code for each declared operation. An abstract class can define an abstract method (that is, define
a type operation without method implementation) by using the abstract keyword in the method declaration.