Java Abstract Classes vs. Interface

What are the differences between the two? Now that Java 8 enables default methods in interfaces, do you even need abstract classes at all?

Lets go over the attributes of an Abstract Class:

• they may or may not include abstract methods
• if a class includes abstract methods, then the class itself must be declared abstract

Now, similarities between Abstract Class and Interface:

• can contain a mix of methods with or without implementation
• both cannot be instantiated

Differences between the two:

Abstract Class:

• can declare fields that are not static and final
• can define public, protected, and private methods
• can only extend one class ( for any class )

Interface:

• all fields are automatically static and final
• methods are automatically public
• methods are implicitly abstract unless declared otherwise
• can implement many interfaces

And when you should consider using each of them:

Abstract Class:

• want to share code among closely related classes
• expect a class that extends an abstract class to have many common fields and methods, or require access modifiers other than public
• want to declare non-static and non-final fields. Also enables you to define methods that can access and modify the state of object to which they belong

Interface:

• expect unrelated classes to implement the interface (e.g interface Comparable and Cloneable are implemented by many unrelated classes)
• want to specify behavior but is not concern about who implements it
• want to take advantage of multiple inheritance

Note that in code, “extends” comes before “implements” or else you will get a compiler error, e.g class A extends B implements C .