What is difference between interface and abstract class in Java ?

Answer 1

Class vs. interface

Some say you should define all classes in terms of interfaces, but I think recommendation seems a bit extreme. I use interfaces when I see that something in my design will change frequently.

interface vs. abstract class

Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.

Answer 2

1. Intetface:- If we don't know anything about implementation just we have requirement specifications (100% Abstraction) then we should go for interface.
for ex. Servlet(I) plan for building

2. Abstract:- If we are taking about implementation but not completely (partial implementation) then we should go for Abstract class.
for ex. GenericServlet & HttpServlet partial complete building.

3. Concreate class:- If we are taking about implementation completely and ready to provide service then we should go for Concreate class.
for ex. MyOwnServlet fully complete building, ready to provide service.