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Abstract classes vs Interfaces: main differences

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Although interfaces and abstract classes are very similar, there are significant differences between them.

Interfaces   Multiple inheritance Implementations allowed Static inner classes

Abstract Classes


  Single inheritance Implementations not allowed Static and non-static inner classes

Interfaces can not be easily modified. Example: a new method is added to an interface but not implemented in a client. A runtime error will occur when non existent method is called.

An abstract class lets to add new methods without breaking compatibility, just provide working implementation of those methods.

When I need to use an Abstract class?

If your API classes that can have more than one implementation from one given class I advise you to use abstract classes, not interfaces. It will be easier to get different implementations.

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