Answer 1

as per dictionary, abstraction is the quality of dealing with ideas rather than functionality.
in java, abstraction is achieved using Abstract classes and interfaces...
sometimes it's useful to declare classes -called abstract classes - for which you NEVER intend to create objects.Because they're used only as superclasses in inheritance hierarchies, we also refer to them as abstract superclasses.they can't be used to instantiate objects because they're INCOMPLETE.subclasses must declare the missing pieces to become CONCRETE.( otherwise subclasses also must be abstract ).purpose of using abstract classes (and interfaces) in program is to share a common design( other classes can inherit them and implement missing piece of them specifically)...