Interface VS Abstract class VS Concrete class?

Answer:
If we don’t know anything about implementation just we have requirement specification (100% Abstraction) then we should go for Interface.
Example: Servlet

If we talking about implementation but not completely (partial implementation) then we go for Abstract class.

Example: GenericServlet and HTTPServlet.
If we are talking about implementation completely and ready to provide service then we should go for concrete class.
Example: MyOwnServlet.

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