为什么抽象类要实现接口？

Question

Since an abstract class can contain both complete and incomplete methods, when is it necessary to implement an interface? When should it implement an interface in an abstract class?

Answer 1

It is a standard way of how OOP works. Imagine a class Human . It is of course abstract as there can not be a concrete instance of a human. A concrete implementation could for example be a class Person that requires a name and some other information.

public class Person extends Human {
    String name;
    int age;
}

A common usage of interfaces is to describe abilities . In our example we could have interfaces like CanWalk , CanBreath , CanJump , NeedsWater , HasGender and so on. In such a case a Human could implement all of these interfaces, it would be perfectly fine.

public abstract class Human implements CanWalk,
    CanBreath, CanJump, NeedsWater, HasGender {
    ...
}

Those interfaces now have methods, like

public interface HasGender {
    String getGender();
}

and Human may implement them, but as an abstract human has no concrete gender, it may delegate the implementation to its implementing class Person :

public class Person extends Human {
    String name;
    int age;
    String gender;

    @Override
    public String getGender() {
        return gender;
    }
}

On the other hand there might be interfaces where Human can offer an implementation, like

public interface NeedsWater {
    int amountOfWaterNeeded();
    void drink(int amount);
}

public abstract class Human implements CanWalk,
    CanBreath, CanJump, NeedsWater, HasGender {

    @Override
    public int amountOfWaterNeeded() {
        return 10;
    }
}

Finally we may have classes that work with interfaces. Like

public class WaterDistributor {
    public void distributeWaterTo(Iterable<NeedsWater> consumers) {
        for (NeedsWater c : consumers) {
            c.drink(c.amountOfWaterNeeded());
        }
    }
}

And you want to be able to pass your humans to that method, so you need to implement the interface.