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Tip of the Day
Language: Design Patterns
Expertise: Beginner
Dec 15, 1999

Interface Usage

Question:
In designing a program, how do I know when to use an interface or a class? Is there a basic rule to follow?

Answer:
Java supports a limited form of multiple inheritance. You may only derive a class from a single superclass, but it may implement as many interfaces as you want. An interface is in essence an abstract class with no implemented methods. It only defines those methods that must be implemented by a subclass.

Therefore multiple inheritance of implementation is not possible in Java, although it may be simulated through aggregation and delegation. Interfaces serve many purposes in Java. You will see them used as a replacement for function pointers, as a mechanism for storing constants that need to be shared by several related classes, and as abstract classes among other things.

In general, you use an interface when each subclass would need to implement the methods differently. Interfaces define common behavior, but not the implementation of that behavior. This allows you to hide classes behind factory patterns that return references to an interface. When a class actually needs to rely on implemented functionality in a superclass, you will not implement the superclass as an interface. Classes allow you to define protected methods that may be accessed by subclasses but not external classes. Interface methods are always public.

It would be a bit disingenuous to say that there is a general rule that dictates when to use an interface or a class. The choice depends on the problem you are trying to solve, the decomposition of functionality, and ultimately your design framework.

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