Java collection classes

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The Collection class HashSet extends the AbstractSet class . Also it implements the Set interface.
But as the AbstractSet class already implements Set interface, then why there is a need for HashSet to implement it again ?

Posted 21-Sep-11 9:01am

snp_shailesh

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-09-21T11:21:00

Solution 1

Nothing wrong here; HashSet implements it in a different way. Interface remains the same, implementation changes, because HashSet features are different. Such re-implementation of some interface methods of base class is quite a usual technique of interface implementation.

—SA

Posted 21-Sep-11 11:21am

Sergey Alexandrovich Kryukov

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snp_shailesh 22-Sep-11 11:27am

Thanks for reply .
I am agree with your explaination.
But still have some generic questions.

As the AbstractSet implements/overrides some methods of Set interface, the same set of methods will be available for its subclass too .
So our HashSet(which extends AbstractSet) can directely overrides its superclass methods in its own featured way .
But instead, HashSet still implements "Set" interface to override the methods. My doubt is, why java has allowed such redundant implementation of interfaces ?