Difference Between Interface and Abstract Class

Difference between Interface and Abstract Class

What is an Abstract Class?

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

What is an Interface?

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class.

So Is there any other major differences ?

Yes…They are as follows:

·         An Abstract class can have a combination of abstract and other normal methods Whereas in Interface it must have only methods (abstract) without definition i.e. Without body.

·         An Abstract class can have any combinations of member variables i.e. public int, private

String, final int etc but in case of Interface all the variables must have default access specifier as public and of type static and final. So Even if you didn’t declare it explicitly it will be always public.

·         As all the member variables are declared public that is why an Interface must also need to

Be declared default as public. While in case Abstract class such conditions are not mandatory.

When to use abstract class and interface in Java

Here are some guidelines on when to use an abstract class and interface in Java:

1.    An abstract class is good if you think you will plan on using inheritance since it provides a common base class implementation to derived classes.

2.    An abstract class is also good if you want to be able to declare non-public members. In an interface, all methods must be public.

3.    If you think you will need to add methods in the future, then an abstract class is a better choice. Because if you add new method headings to an interface, then all of the classes that already implement that interface will have to be changed to implement the new methods. That can be quite a hassle.

4.    Interfaces are a good choice when you think that the API will not change for a while.

5.    Interfaces are also good when you want to have something similar to multiple inheritance, since you can implement multiple interfaces.