使用抽象类而不是策略设计模式中的接口

Question

I'm just learning Java and design patterns and I am trying to get my head around when to use interfaces and abstract classes. I am wondering in the Strategy design pattern, why is it preferable to use an interface for the behaviour/algorithm subclasses rather than an abstract superclass?

Is it simply because there is no need for an abstract class because each behaviour/algorithm subclasses have their own implementation and therefore an abstract superclass would just provide extra functionality that won't be used?

Wouldn't an abstract class mean that there is an extra possibilty to use this extra functionality in the future if needed, for instance adding a method to the abstract superclass which is shared by the behaviour/algorithm subclasses if this is needed. Is there any reason why this is a bad idea?

Or is there another reason?

Answer 1

You should use a common interface for all your strategies and also create an abstract class that implements it. So, although all your strategies share an abstract class currently, your system is extendable because you can create unrelated strategies to this abstract one in the future.

Now using only some methods of the superclass is a decision you should make. It may be worthy or not.

note:: If you are using something similar to the Template Method Pattern you should check how to use hooks.

Answer 2

Before Java 8, interfaces had one big issue: it was impossible to add additional functionality (ie add new methods) without breaking every implementation of that interface. Abstract base classes were a way to avoid this problem.

Java 8 introduced default methods , which mostly addresses that issue. So with a modern Java system, interfaces provide much more flexibility without the downside.

Answer 3

From an implementation point of view you can use both, interface and abstract class. The difference is in the purpose of use.

• The main purpose of interfaces is to provide methods (ie communication points) to other classes. Ie decoupling and inversion of dependency .
• The main purpose of abstract classes is to help you build an inheritance structure upon them.

In the strategy pattern that means, if you use an AbstractState to define a common code base, you could still put an IState interface between the Context and the AbstractState to decouple them more.

Answer 4

There's no hard rule between using an interface or an abstract pattern. Use which ever is best in your situation.

If you have multiple Strategy implementations that are very similar of course you can use abstract classes or even subclass one of the Strategies.