Do/can abstract classes replace interfaces? [duplicate]

Answer 1

Not always:

• a class can extend only one class
• a class can implement more than one interface

Sun docs make a more detailed comparison:

Abstract Classes versus Interfaces

Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.

Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable, for example.

By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences (the abstract methods).

Answer 2

Have a look at the interesting article "Why extends is evil" to get an idea about the differences between interface implementation and class inheritance (beside the obvious multi- single restrictions)

Answer 3

Abstract classes and interfaces are complementary.

For instance when creating an API you will want to present interfaces to the client, so that you may always completely change the implementation whereas he does not have to change its code and the user does not rely on implementation when building using your API but just on methods contracts.

Then you will have abstract classes partly implementing these interfaces, in order to

• share some common code, which might be used in all (or almost all) implementations for interface, which is obvious
• provide default behaviour which could be overridden in 'real' implementations, for instance a toString() method using interfaces methods to create a textual representation of the implementation
• preserve implementations compatibility after interface changes, for instance when you add a new method in your interface, you also add a default implementation in the abstract class so that implementations (for instance those made by the user) extending the abstract class still work without changes

Answer 4

Interfaces are much cleaner and light weight. Abstract classes make you dependent on it heavily as you cannot extend any other classes.

Answer 5

Main use of interface is when you create the reference of interface and call the method of particular class that's resolved at runtime. So it's always better idea to create reference of interface to call the method.

Answer 6

Interfaces can only hold abstract method, also interfaces can implement multiple interfaces to any class. But an abstract hold abstract and non abstract methods, and abstract methods cannot extend to more then one class.