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why most classes and interface in servlet package abstract ?

 
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Please can any one tell me why most classes and interface in servlet package abstract
 
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It is obvious that interfaces must be abstract.

Apart from that, only GenericServlet and HttpServlet classes are abstract (if I am not wrong), so they must be subclassed to create an (HTTP)servlet.

For example, to write a generic servlet, you need only override the abstract service method, because the rest of the methods are already implemented in the GenericServlet class.
 
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Interfaces are "rules" so they must be present in specifications implemented by third parties as is the case with servlets and JSP.

Abstract classes let you create your own classes "extending" the abstract functionality. Moreover, they ease the developer by providing dummy method implementations.
 
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