if a class contains abstract method,that class must be
declared as a abstract class. abstract methods contains only
declarations. it does not contain objects.

In Inheritance , class become more general and more
abstract.Ancestor classes are more general, i.e more basis
for other classes.Abstract classes can have fields and
concrete methods.Methods are declared not defined so we can
not create objects. but we can create object variables of an
abstract class. but such variable must refer to an object of
a nonabstract subclass.