What are the differences between Interface and Abstract class?

Abstract Classs	Intrerface
A class may extend only one abstract class.	A Class may implement several interfaces.
An abstract class can have non-abstract methods.	All methods of an Interface are abstract.
An abstract class can have instance variables.	An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.	An Interface visibility must be public (or) none.
An abstract class can contain constructors.	An Interface cannot contain constructors.
An abstract class can provide complete, default code and/or just the details that have to be overridden.	An interface cannot provide any code at all, just the signature. (I.e. All Interface methods are public).
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.