Do/can abstract classes replace interfaces? [duplicate]

Answer 1

To answer your question, yes you could use an abstract class (providing no implementation) instead of an interface but I'd consider this bad practice:

• You've used up your "one-shot" at inheritance (without gaining any benefit).
• You cannot inherit from multiple abstract classes but you can implement multiple interfaces.

I would advocate the use of abstract classes more in situations where you wish to provide a partial implementation of a class, possibly delegating some behavior to concrete subclass implementations.

Answer 2

In some cases you can use an abstract class instead of interface. However, it is hardly ever a good idea to do so. In general you should use the rule:

1. Interfaces specify behaviour.
2. Abstract classes specify implementation.

The other "problem" with using abstract classes is that you can then no longer implement mixins, that is you can implement multiple interfaces, however you can only extend one abstract class.

Answer 3

One point missing from the answers here is the idea of who will be implementing the interface.

If your component wants to return instances of abstract types to its callers, where the concrete types are defined internally and hidden from callers, use an interface. Conversely, if your component consumes or accepts instances of abstract types that its callers must implement, abstract classes are usually a better choice.

Anticipating evolution and maintaining binary compatibility tips the scales here. With an abstract class, you can add methods and, if you provide a base implementation, existing implementations of the abstract class will continue to work fine. With an interface, adding a method breaks binary compatibility, for no existing implementation could possibly continue to compile properly without changing to define the new method.

The Apache Cactus project has a good discussion on how to resolve these obligations.

Answer 4

• A class in java can inherit from multiple interfaces, but only from one abstract class.

• An interface cannot define any code, in an abstract class, you can define code (i.e. default behaviour of methods)

Answer 5

Abstract classes are the partial implementation of Abstraction while Interfaces are the fully implementation of Abstraction.Means in Abstract classes we can put methods declaration as well as method body. We can't create an object of Abstract classes(association) and reuse the class by inheritence(not by association). By default in interfaces all declared variables are static final and All methods are public.

For Example: In JDK there are only few abstract classes and HttpServlet is one of them which is used in Servlet.So we can't create object of HttpServlet and it can be used only by inheritence.