What is the difference between interface, abstract class and concrete class?

When we should go for interface, abstract class and concrete class?

Interface, Abstract class and Concrete class

1.If we don't know anything about implementation just we have requirement specification then we should go for interface.

2.If we are talking about implementation but not completely (partial implementation) then we should go for abstract class.

3.If we are talking about implementation completely and ready to provide service then we should go for concrete class.

Interface	Abstract class
If we don't' know anything about implementation just we have requirement specification then we should go for interface.	If we are talking about implementation but not completely (partial implementation) then we should go for abstract class.
Every method present inside interface is always public and abstract whether we are declaring or not.	Every method present inside abstract class need not be public and abstract.
We can't declare interface methods with the modifiers private, protected, final, static, synchronized, native, strictfp.	There are no restrictions on abstract class method modifiers.
Every interface variable is always public static final whether we are declaring or not.	Every abstract class variable need not be public static final.
Every interface variable is always public static final we can't declare with the following modifiers. Private, protected, transient, volatile.	There are no restrictions on abstract class variable modifiers.
For the interface variables compulsory we should perform initialization at the time of declaration otherwise we will get compile time error.	It is not require to perform initialization for abstract class variables at the time of declaration.
Inside interface we can't take static and instance blocks.	Inside abstract class we can take both static and instance blocks.
Inside interface we can't take constructor.	Inside abstract class we can take constructor.