Interface and Abstract class

Interface vs. Abstract class

• Interfaces provide a form of multiple inheritance.
• A class can extend only one other class.

• Interfaces are limited to public methods and constants with no implementation.
• Abstract classes can have a partial implementation, protected parts, static methods, etc.

• A Class may implement multiple interfaces.
• In case of abstract class, a class may extend only one abstract class.

• Interfaces are slower as it requires extra indirection to to find corresponding method in in the actual class.
• Abstract classes are fast.

Similarities

• Neither Abstract classes or Interface can be instantiated

When to use Interface or Abstract class ?

Abstract class

• The advantage of an abstract class is that you can (partially) implement the class.
• Use abstract Java classes when you want to provide some standard base code but want / need to force the user's of your class to complete the implementation.

Interface

• If you have a group of classes that you want to call the same methods on; just make them implement the interface and you will be able to program to the interface, rather than one of the individual classes.
• The advantage of an interface is that you can implement multiple interfaces, whereas you can not extend multiple abstract classes.

"From a design perspective, I tend to favor Interfaces over abstract classes.

They ensure you are not tied to a specific implementation, are simpler to work with when things get complex, and allow you to add to your application by building new classes that implement the interface."                

- Shahnawaz Khan (Blog Author)