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Why we need both interface and abstract class in java?

 
Greenhorn
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I am sorry if this question has been asked many a time.
But my question is when to use an interface and when an abstract class. Can one can ubstitute the other? Does interface help us achieve multiple inheritance? Elaborate answers will be appreciated.
 
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Welcome to JavaRanch

It is by no means a new question, but interfaces allow for more flexibility than abstract classes. An abstract class (often) has part of the implementation already set up, requiring only minor changes to implement fully.
An interface has none of its implementation, so it requires more coding to implement, but each implementation can interpret the specification differently.

And interfaces allow the Java programmer to mimic multiple inheritance, yes.
 
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