抽象类和接口的比较

An abstract class is a class that is only partially implemented by the programmer. It may contain one or more abstract methods. An abstract method is simply a function definition that serves to tell the programmer that the method must be implemented in a child class.
抽象类，是没有实现“完全”的类，里面包含了一个或者多个抽象方法，这些抽象方法需要子类实现。所以抽象类一定是要被继承的，不能实例化。

An interface is similar to an abstract class; indeed interfaces occupy the same namespace as classes and abstract classes. For that reason, you cannot define an interface with the same name as a class. An interface is a fully abstract class; none of its methods are implemented and instead of a class sub-classing from it, it is said to implement that interface.

接口“类似于”一个完全抽象的class，所有方法都没有实现。但是接口本身并不是类，这里只是“类似于”。

An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not have a body. The interface can’t do anything. It’s just a pattern.

For example (pseudo code):

// I say all motor vehicles should look like this:interface MotorVehicle{    void run();    int getFuel();}// My team mate complies and writes vehicle looking that wayclass Car implements MotorVehicle{    int fuel;    void run()    {        print("Wrroooooooom");    }    int getFuel()    {        return this.fuel;    }}

Implementing an interface consumes very little CPU, because it’s not a class, just a bunch of names, and therefore there isn’t any expensive look-up to do. It’s great when it matters, such as in embedded devices.

Abstract classes, unlike interfaces, are classes. They are more expensive to use, because there is a look-up to do when you inherit from them.

Abstract classes look a lot like interfaces, but they have something more: You can define a behavior for them. It’s more about a guy saying, “these classes should look like that, and they have that in common, so fill in the blanks!”.

// I say all motor vehicles should look like this:abstract class MotorVehicle{    int fuel;    // They ALL have fuel, so why not let others implement this?    // Let's make it for everybody.    int getFuel()    {         return this.fuel;    }    // That can be very different, force them to provide their    // implementation.    abstract void run();}// My teammate complies and writes vehicle looking that wayclass Car extends MotorVehicle{    void run()    {        print("Wrroooooooom");    }}

所以，
抽象类更像是对一系列类的抽象，这些类的共性(相同点)提取出来在抽象类中实现，而不同点在抽象类中不实现，等待具体的子类去实现，这样是不是省了不少代码^_^。
而接口更像是为了面向接口编程而实现的，一个组织(类似于sun)提出了一个标准，这些标准其实可以看做一系列接口，sun并不去具体实现这些“功能”，而第三方负责实现这个标准，例如apache tomcat，将“标准”和“实现”分开。