Question

1. What is use of an abstract class implementing an interface? In which
   scenario would we implement.
2. Why would you choose Abstract class over interface? If an class
   extends an Abstract class, should we implement all the methods in
   the Abstract Class. If an class implements an Interface should we
   implement all the methods too.
3. Why was abstract class or interface pattern introduced? What is the
   use of them? Does it deal anything in the way object is getting
   instantiated or the way it behaves?
4. If an abstract class has a static method declared, then can we
   instantiate that class?

I was asked these questions, though my answers were not clear… i would like to know from people here?

Answer 1

On Abstract classes and Interfaces

Abstract classes defines a partial representation of some entity, that is common to every extending class but that must be completed by inheritance. So, every inheriting class comprehends all the properties defined in every parent classes.

Interfaces provide a behavior to a given class, and allow to simulate something similar to multiple inheritance. Since you can implement multiple interfaces in a class, you can make that class adopt the behavior of every implementing interface, and all together.

Classical example:
Abstract class: Animal Represents an animal in an abstract way. Each animal should extend this class to adopt all the implications of “being” an animal.

Abstract class: Mammal extends Animal The mammal is an Animal and inherits all the properties that are common to every mammal.

Some mammals are carnivorous and some herbivorous. These are different behaviors for the same type of animal, so here they come the interfaces.

Interface: Carnivorous Defines the properties a carnivore animal should have.

Interface: Herbivorous Defines the properties a herbivore animal should have.

Every mammal should breath air. This is another behavior, that is common to every mammal.

Interface AirBreathing Defines the properties for air-breathing animal

So. You have these two mammals: Wolves and Manatees. Both have to breath air, but wolves are carnivorous and manatees herbivorous.

Class Wolf extends Mammals implements AirBreathing, Carnivorous

Class Wolf extends Mammals implements AirBreathing, Herbivorous

Implementation details

1) Every abstract method defined in an abstract class, must be implemented somewhere “in the road” of inheritance until a final class is reached. This means that when a final class is implemented it must implement or inherit implementations of every abstract method inherited from parent classes.

2) Yes, you can define static methods in abstract classes. You can call static method from the abstract class as AbstractClass.staticMethod(), but you can’t ever instantiate an abstract class.

3) Classes implementing interfaces, must implement every method defined by the interface.

Hope this helps.