一旦你需要为你的类强制一个行为，你应该定义一个接口。

动物的行为可能包括走、吃、跑等。因此，您将它们定义为接口。

另一个实际的例子是ActionListener(或Runnable)接口。您可以在需要跟踪特定事件时实现它们。因此，您需要在类(或子类)中提供actionPerformed(Event e)方法的实现。类似地，对于Runnable接口，提供公共void run()方法的实现。

此外，您可以让任意数量的类实现这些接口。

使用接口(在Java中)的另一个实例是实现c++中提供的多重继承。

当相同功能的实现不同时使用接口。

当你需要共享一个公共的具体实现时，使用一个抽象/基类。

一些非编程示例可能帮助您了解接口在编程中的适当使用。

There's an interface between electrical devices and the electricity network - it's the set of conventions about the shape of the plugs and sockets and the voltages/currents across them. If you want to implement a new electrical device, as long as your plug follows the rules it will be able to get services from the network. This makes extensibility very easy and removes or lowers the costs of coordination: you don't have to notify the electricity provider about how your new device works and come to a separate agreement about how to plug your new device into the network.

各国都有标准的铁路轨距。这使得铺设铁轨的工程公司和建造在这些铁轨上运行的列车的工程公司之间实现了分工，并使铁路公司有可能在不重新设计整个系统的情况下更换和升级列车。

The service a business presents to a client can be described as an interface: a well defined interface emphasises the service and hides the means. When you put a letter in a mailbox, you expect the postal system to deliver the letter within a given time but you have no expectations about how the letter is delivered: you don't need to know, and the postal service has the flexibility to choose the means of delivery that best meets the requirements and current circumstances. An exception to this is the ability of customers to choose airmail - that's not the kind of interface a modern computer programmer would have designed, since it reveals too much of the implementation.

来自自然的例子:我不太喜欢eats()， makesSound()， moves()等例子。它们确实描述了行为，这是正确的，但它们没有描述交互以及它们是如何被启用的。在自然界中，使相互作用成为可能的界面的一个明显的例子是与繁殖有关的，例如一朵花为蜜蜂提供了一个特定的界面，以便授粉能够发生。

假设你正在制作一款第一人称射击游戏。玩家有多种枪可供选择。

我们可以有一个接口Gun，它定义了函数shoot()。

我们需要不同的子类枪类，即霰弹枪狙击手等。

ShotGun implements Gun{
    public void shoot(){
       \\shotgun implementation of shoot.
    } 
}

Sniper implements Gun{
    public void shoot(){
       \\sniper implementation of shoot.
    } 
}

射击类

射手把所有的枪都装在他的盔甲里。让我们创建一个List来表示它。

List<Gun> listOfGuns = new ArrayList<Gun>();

射手在需要时使用switchGun()函数循环使用他的枪。

public void switchGun(){
    //code to cycle through the guns from the list of guns.
    currentGun = //the next gun in the list.
}

我们可以使用上面的函数设置当前的Gun，当调用fire()时，简单地调用shoot()函数。

public void fire(){
    currentGun.shoot();
}

shoot函数的行为将根据Gun接口的不同实现而有所不同。

结论

当一个类函数依赖于来自另一个类的函数时，创建一个接口，而另一个类根据实现的类的实例(对象)改变其行为。

例如，Shooter类的fire()函数期望枪械(Sniper, ShotGun)实现shoot()函数。 所以如果我们换枪开火。

shooter.switchGun();
shooter.fire();

我们已经改变了fire()函数的行为。

In my experience the driving force to create interfaces didn't occur until I start doing unit testing with a mocking framework. It became abundantly clear that using interfaces was going to make mocking much easier (since the framework depended on the methods being virtual). Once I started I saw the value of abstracting away the interface to my class from the implementation. Even if I don't create an actual interface, I try now to make my methods virtual (providing an implicit interface that can be overridden).

我发现还有许多其他原因可以加强重构到接口的良好实践，但是单元测试/模拟的事情提供了最初的“顿悟时刻”的实践经验。

EDIT: To clarify, with unit testing and mocking I always have two implementations -- the real, concrete implementation and an alternate mock implementation used in testing. Once you have two implementations, the value of the interface becomes obvious -- deal with it in terms of the interface so you can replace the implementation at any time. In this case I'm replacing it with a mock interface. I know that I can do this without an actual interface if my class is constructed properly, but using an actual interface reinforces this and makes it cleaner (clearer to the reader). Without this impetus, I don't think I would have appreciated the value of interfaces since most of my classes only, ever have a single concrete implementation.

它还允许您执行模拟单元测试(. net)。如果您的类使用接口，您可以在单元测试中模拟对象，并轻松地测试逻辑(无需实际触及数据库或web服务等)。

http://www.nmock.org/