Why to use Interfaces, Multiple Inheritance vs Interfaces, Benefits of Interfaces?

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Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.

We can’t. Interfaces aren’t used to achieve multiple inheritance. They replace it with safer, although slightly less powerful construct. Note the keyword implements rather than extends.

Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?

They are not. With interfaces a single class can have several “views“, different APIs or capabilities. E.g. A class can be Runnable and Callable at the same time, while both methods are effectively doing the same thing.

Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.

Interfaces are kind-of multiple inheritance with no problems that the latter introduces (like the Diamond problem).

There are few use-cases for interfaces:

  1. Object effectively has two identities: a Tank is both a Vehicle and a Weapon. You can use an instance of Tank where either the former or the latter is expected (polymorphism). This is rarely a case in real-life and is actually a valid example where multiple inheritance would be better (or traits).

  2. Simple responsibilities: an instance of Tank object in a game is also Runnable to let you execute it in a thread and an ActionListener to respond to mouse events.

  3. Callback interfaces: if object implements given callback interface, it is being notified about its life-cycle or other events.

  4. Marker interfaces: not adding any methods, but easily accessible via instanceof to discover object capabilities or wishes. Serializable and Cloneable are examples of this.

What you are looking for are trait (like in Scala), unfortunately unavailable in Java.

More Related Contents:

  1. Why is there no multiple inheritance in Java, but implementing multiple interfaces is allowed?
  2. What is an interface in Java?
  3. How should I have explained the difference between an Interface and an Abstract class?
  4. When do I have to use interfaces instead of abstract classes? [duplicate]
  5. How can interfaces replace the need for multiple inheritance when have existing classes
  6. How much interfaces a class file can implement [closed]
  7. What’s wrong with overridable method calls in constructors?
  8. Difference between Inheritance and Composition
  9. Do subclasses inherit private fields?
  10. Abstract class in Java
  11. Java – Method name collision in interface implementation
  12. Explicitly calling a default method in Java
  13. Why java classes do not inherit annotations from implemented interfaces?
  14. What is the main difference between Inheritance and Polymorphism?
  15. How do Java Interfaces simulate multiple inheritance?
  16. Why do we assign a parent reference to the child object in Java?
  17. Is polymorphism possible without inheritance?
  18. Can an interface extend multiple interfaces in Java?
  19. Why are class static methods inherited but not interface static methods?
  20. Do/can abstract classes replace interfaces? [duplicate]
  21. How can a derived class invoke private method of base class?
  22. Should you always Code To Interfaces In Java [closed]
  23. Why would both a parent and child class implement the same interface?
  24. Access subclass fields from a base class in Java
  25. Are defaults in JDK 8 a form of multiple inheritance in Java?
  26. interface as a method parameter in Java
  27. Anonymous vs named inner classes? – best practices?
  28. Why are interfaces preferred to abstract classes?
  29. The purpose of interfaces continued
  30. Multiple Inheritance in java

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