带通配符的泛型（地图 <? extends A, List<? extends B> &gt;）

Question

Consider the following code:

public interface A {};

public class AImpl implements A {};

public interface B {};

public class BImpl implements B {};

public interface Service{
  Map<? extends A, List<? extends B>> get();
}

Why does the following implementation of Service not compile?

public class ServiceImpl implements Service {
  public Map<AImpl, List<BImpl>> get() {
    return null;
  }
}

Compiler error:

The return type is incompatible with Service.get()

But the following code compile:

public interface Service{
   List<? extents B> get();
}

public class ServiceImpl implements Service{
   public List<BImpl> get(){
       return null;
   }
}

Answer 1

Because <? extends BaseType> <? extends BaseType> means "some unspecified sub-type BaseType", and class Sub extends BaseType , while sub-type of BaseType , is not it. Read Java Generics FAQ , in particular starting from Wildcard Capture section, for more details.

You should generify your code properly:

public interface Service<K extends A, V extends B> {
    Map<K, List<V>> get();
}

public class ServiceImpl implements Service<AImpl, BImpl> {
    @Override
    public Map<AImpl, List<BImpl>> get() {
        return null;
    }
}

Answer 2

ServiceImpl cannot be an interface because your Implementation cannot be in an interface; Change it to a base base class and try it.

Answer 3

What do you write?

Map<? extents A, List<? extents B> get();

java doesn't know anything about extents

At least use extends

Answer 4

The return type of the Service#get() method is specified as

Map<? extends A, List<? extends B>> get();

And you are trying to return a

Map<AImpl, List<BImpl>> 

You can use covariant return types. And it seems like you thought this would be the case here. But the problem is that this covariance does not apply to the generic type parameters. Although List<BImpl> is a subtype of List<? extends B> List<? extends B> , this does not mean that Map<AImpl, List<BImpl>> is a subtype of Map<? extends A, List<? extends B>> Map<? extends A, List<? extends B>> Map<? extends A, List<? extends B>> .

A structurally similar but simpler case is that List<Integer> is not a subtype of List<Number> .

You could change the return type in the Service interface to

Map<? extends A, ? extends List<? extends B>> get();

to make it work.