When to implement an interface and when to extend

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 really compares the two side by side and explains when and why you would want to use one or the other. I have not found a lot of times when implementing an interface would be a better system than extending a superclass.

So when do you implement an interface and when do you extend a superclass?

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

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 3

I guess I'll give the classic car example.

When you have a car interface, you can create a Ford, a Chevy, and an Oldsmobile. In other words, you create different kinds of cars from a car interface.

When you have a car class, you can then extend the car class to make a truck, or a bus. In other words, you add new attributes to the sub classes while keeping the attributes of the base or super class.

Answer 4

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 5

You can think of extending from a super class if the derived class is of the same type.I mean that when a class extends an abstract class, they both should be of the same type, the only difference being that the super class has a more general behavior and the sub class has a more specific behavior. An interface is a totally different concept. When a class implements an interface, its either to expose some API(contract) or to get certain behavior. To give an example, I would say that Car is an abstract class. You can extend many classes from it like say Ford, Chevy and so on which are each of type car. But then if you need certain specific behavior like say you need a GPS in a car then the concrete class, eg Ford should implement GPS interface.

Answer 6

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.