What is use of Parent object instantiating with child class?

This is the basis for polymorphism : Imagine you have several child classes that inherit from you parent class. You want to use all these child classes through the interface / methods defined on your parent class, without worrying about the implementation details in each child class (each might do something different, but with the same overall semantics).

In your example, B is-a A, but A is-not-a B.

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