Java：与实现接口的类不兼容的返回类型

Question

I have an interface and an implementation:

// there are things
public interface Thing { /* ... */ }

// balls are things
public class Ball implements Thing { /* ... */ }

I have another interface and another implementation:

// a dog can fetch any thing
public interface Dog {
    public Thing fetch();
}

// a shepherd can fetch only a ball
public class Shepherd implement Dog {
    public Ball fetch() { /* ... */ }
}

This is fine.

But now I want to use Ball as a parameter of a parametrized class:

public interface Dog {
    public List<Thing> fetchList();
}

public class Shepherd implement Dog {
    public List<Ball> fetchList() { /* ... */ }
}

Java doesn't allow me to do so, it says I'm attempting to use incompatible type.

At the same time this construction works without any issues:

public class Shepherd implement Dog {
    public List<Thing> fetchList() { /* ... */ }
}

Why does Java allow me to use an implementation of an interface as the returning value's definition, but doesn't allow me to use it as the parametrized class' parameter?

Thanks in advance!

Answer 1

Because not every Thing is a is a Ball

You can try to parametrize super type like this

public interface Dog<T extends Thing> {
    public List<T> fetchList();
}

public class Shepherd implement Dog<Ball> {
    public List<Ball> fetchList() { /* ... */ }
}