Question

Is it really necessary to implement all the methods to a subclass(inherited from a abstract class), if there is another subclass of that abstract class has already implemented those abstract methods?

abstract class A {
    abstract void method();
    abstract void anothermethod();
}

class B extends A {   
    void method() {}
    void anothermethod() {}    
}

class C extends A { // is this class definition is legal?
    void sample() {}    
}

Answer 1

No, it’s not legal. You’ve got a concrete class (C is not declared abstract) extending an abstract class, but without providing implementations for its methods. It’s not really clear why you think this should or could be legal – and you should consider whether this has wider ramifications for your understanding of inheritance in general than just this specific case.

C is entirely separate from B. They could have entirely different state – for example, B might implement method() using some state which is only present in an instance of B. It’s important to understand that an instance of C is not an instance of B.

If you want it to inherit its behaviour, you should make C subclass B instead of A.