Ans: Abstraction and Polymorphic are two important principales of OOPS.
Abstraction : It means hidding the implementation details from public interface. Public interface means that eg: If your code or class is using by another programmers, you can change the implementation without braking the code of others .It provide full flexibility and maintainability to your code.
other example: you can use abstraction by making instance variable private and method public . you can altering the way of using variable without change the way of call your method by others.\\
Polymorphism: Poly+morphism means that many+form. In layman language ,one human being plays more than one role in single life like (husband,son etc).
In java all java objects are polymorphic because they pass the IS-A
test for their own type and class object.
example: single java object may object, animal,
fruit.