Inheritance (the subclass relationship)
In the Java class header, we put B extends A which means B is a subclass of A, if B has all the variables and methods of A (and more).
In the class definition of B, the child class, there is no need to repeat declarations of variables and methods of A, they are assumed to be there. The definition of B has the additional variables and methods of B.
Or B may give a new definition of some method of A. This is called overriding.