title: Why does all the interface methods need to be implemented in a class implementing it in java
body: <p>I know that it is the purpose of the interface and the class can be declared abstract to escape from it. </p>
  
  <p>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?</p>