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I know that it is the purpose of the interface and the class can be declared abstract to escape from it.
But is there any use for implementing all the methods that we declare in an interface? will that not increase the weight and complexity of the code if we keep on defining all the methods even it is not relevant for that class? why it is designed so?
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It is used to provide total abstraction. That means all the methods in an interface are declared with an empty body and are public and all fields are public, static, and final by default. A class that implements an interface must implement all the methods declared in the interface.
Yes, it is mandatory to implement all the methods in a class that implements an interface until and unless that class is declared as an abstract class.
If you don't implement all methods of your interface, than you destroy the entire purpose of an interface.
Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler.
The idea of an interface in Java is very much like a contract (and perhaps seen in retrospect this should have been the name of the concept)
The idea is that the class implementing the interface solemnly promises to provide all the things listed in the contract so that any use of a class implementing the interface is guaranteed to have that functionality available.
In my experience this facility is one of the things that makes it possible to build cathedrals in Java.
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