pure abstract class and interface [duplicate]

Answer 1

To complete the former answers :

An interface is a "contract". If a class implements an interface it have to propose all the services listed in the interface.

An abstract class is a skeleton. It defines a certain way its extended classes will work while letting them some free space (the abstract methods) to be unique.

A pure abstract class doing the same thing as a interface but have the problem of unique extending so, for me, it have no interest

Answer 2

I'm just going to address one point (mainly because the other questions have been addressed already):

"Where interface is being used we can make use of pure abstract class?"

In theory, you could. However, you will lose flexibility and loose coupling to some extent. It's far more preferable to code to interfaces and pass those around, especially in Inversion of Control (IoC) scenarios and from an integration point of view, as this allows far greater extensibility.

Answer 3

There are three differences:

1. Interfaces can only declare public methods (i.e. no protected or package-private visible methods) and can not declare any fields
2. Subclasses can only extend at most one abstract class, but can implement any number of interfaces
3. The abstract class can also have implementations for some or all of the methods

Answer 4

An abstract class can provide an implementation, i.e. (public, protected, private) method bodies. An interface can just declare public method signatures. These methods have to be realized (in the form of method bodies) by classes implementing the interface.

Answer 5

One reason to choose pure abstract over interface is to force sub classes to implement particular methods that are implemented by a super class.

For example (in Java),

Say you want all extending classes to implement toString(), equals(), and hashCode().

You could create an interface called ForceSomeMethods for that contract, but those methods are implicitly implemented by Object.

Making ForceSomeMethods a pure abstract class with toString(), etc as abstract methods, all subclasses will be forced to implement those methods.