When to implement an interface and when to extend a superclass?

Question

I\'ve been reading a lot about interfaces and class inheritance in Java, and I know how to do both and I think I have a good feel for both. But it seems that nobody ever rea

Answer 1

One method of choosing between an interface and a base class is the consideration of code ownership. If you control all the code then a base class is a viable option. If on the other hand many different companies might want to produce replaceable components, that is define a contract then an interface is your only choice.

Answer 2

Use an interface to define behavior. User (abstract) classes (and subclasses) to provide implementation. They are not mutually exclusive; they can all work together.

For example, lets say you are defining a data access object. You want your DAO to be able to load data. So put a load method on the interface. This means that anything that wants to call itself a DAO must implement load. Now lets say you need to load A and B. You can create a generic abstract class that is parameterized (generics) to provide the outline on how the load works. You then subclass that abstract class to provide the concrete implementations for A and B.

Answer 3

The main reason for using abstract classes and interfaces are different.

An abstract class should be used when you have classes that have identical implementations for a bunch of methods, but vary in a few.

This may be a bad example, but the most obvious use of abstract classes in the Java framework is within the java.io classes. OutputStream is just a stream of bytes. Where that stream goes to depends entirely on which subclass of OutputStream you're using... FileOutputStream, PipedOutputStream, the output stream created from a java.net.Socket's getOutputStream method...

Note: java.io also uses the Decorator pattern to wrap streams in other streams/readers/writers.

An interface should be used when you just want to guarantee that a class implements a set of methods, but you don't care how.

The most obvious use of interfaces is within the Collections framework.

I don't care how a List adds/removes elements, so long as I can call add(something) and get(0) to put and get elements. It may use an array (ArrayList, CopyOnWriteArrayList), linked list (LinkedList), etc...

The other advantage in using interfaces is that a class may implement more than one. LinkedList is an implementation of both List and Deque.

Answer 4

I found some articles, particularly some who describe why you should not use implementation inheritance (i.e. superclasses):

• Why extends is evil
• Inheritance of implementation is evil
• Implementation inheritance
• Implementation inheritance
• Java inheritance FAQ

Answer 5

If you only want to inherit method signatures (name, arguments, return type) in the subclasses, use an interface, but if you also want to inherit implementation code, use a superclass.