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What are the differences between Interface and Abstract class?

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.
In case of abstract class, 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.
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.
An abstract class can contain constructors .
An Interface cannot contain constructors .
Abstract classes are fast.
Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

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