-only be associated with methods and classes.
-if a method is abstract, it means we are declaring a method without body.
-if a method is abstract, its class should be declared as abstract.
-abs class can’t be instantiated.
-abs class/method is meaningful only in the context of inheritance.
-top classes in the hierarchy are declared as abstract.
-abs class can have non abs methods.
-but reference can be created.
