What do you mean when you say that Java has two concepts?

Answer 1

You are talking about the implementation point view of Abstract class and the interface. Let's go. 1. Interface helps Multiple inheritance:- In java you can't have a class inherited from more than one class, i.e. the multiple inheritance. Interface helps us in implementing the multiple inheritance(virtually), because their is no such restriction in implementing more than one interface. But the Abstract class can't help you doing so. 2. Interface helps global declaration:- In C/C++, a header file holds the declaration of the functions and the symbolic constants. Similarly we can have a interface which is having only the symbolic constants which is same for all classes which implements it. And also the method have only the signature and the class which implements it is free to write the body at his own wish. Abstract class can't help in doing this. 3. Abstract class is more secure:- Because in Interface we have all the things public. But in Abstract class we can have private and also friendly members. 4. Interface is overhead:- Because in Interface we are bound to implement all the methods though some of them we actually need. But in case of the Abstract class we can override the methods which we want.