What is the difference between an interface and abstract class?

Question

What exactly is the difference between an interface and abstract class?

Answer 1

I am constructing a building of 300 floors

The building's blueprint interface

• For example, Servlet(I)

Building constructed up to 200 floors - partially completed---abstract

• Partial implementation, for example, generic and HTTP servlet

Building construction completed-concrete

• Full implementation, for example, own servlet

Interface

• We don't know anything about implementation, just requirements. We can go for an interface.
• Every method is public and abstract by default
• It is a 100% pure abstract class
• If we declare public we cannot declare private and protected
• If we declare abstract we cannot declare final, static, synchronized, strictfp and native
• Every interface has public, static and final
• Serialization and transient is not applicable, because we can't create an instance for in interface
• Non-volatile because it is final
• Every variable is static
• When we declare a variable inside an interface we need to initialize variables while declaring
• Instance and static block not allowed

Abstract

• Partial implementation
• It has an abstract method. An addition, it uses concrete
• No restriction for abstract class method modifiers
• No restriction for abstract class variable modifiers
• We cannot declare other modifiers except abstract
• No restriction to initialize variables

Taken from DurgaJobs Website