泛型：尝试将参数化集合传递给List addAll方法

Question

I have code (full source code):

public class AutoConversionTest {

    @Test
    public void test_autoConversion() {
        Wrapper wrapper = new Wrapper();
        wrapper.setList(new ArrayList<Sub>());
        wrapper.addAll(new ArrayList<Sub>());
    }

    class Wrapper {
        List<? extends Super> list;

        public void setList(List<? extends Super> list) {
            this.list = list;
        }

        public void addAll(List<? extends Super> list) {
            this.list.addAll(list);  //TROUBLES!
        }
    }

    class Super {}

    class Sub extends Super {}
}

Question : Wy error and how to sovle it?

EDITED : my error log

java: no suitable method found for `addAll(java.util.List<? extends expectations.public_method.experiments.AutoConversionTest.Super>)`
 method `java.util.List.addAll(int,java.util.Collection<? extends expectations.public_method.experiments.AutoConversionTest.Super>)` is not applicable (actual and formal argument lists differ in length)
 method `java.util.List.addAll(java.util.Collection<? extends expectations.public_method.experiments.AutoConversionTest.Super>)` is not applicable (actual argument `java.util.List<? extends expectations.public_method.experiments.AutoConversionTest.Super>` cannot be converted to `java.util.Collection<? extends expectations.public_method.experiments.AutoConversionTest.Super>` by method invocation conversion)

Answer 1

List<? extends Super> List<? extends Super> means: a list of some unknown type, which extends Super. So, you can't add anything (except null) to such a list, since you don't know the type of its elements. If this was allowed by the compiler, you could add instances of OtherSub to a List<Sub> , which would ruin the type-safety of the list.

Change the type of Wrapper.list to List<Super> .

EDIT:

Modified code:

class Wrapper {
    List<Super> list;

    public void setList(List<? extends Super> list) {
        this.list.clear();
        this.list.addAll(list);
    }

    public void addAll(List<? extends Super> list) {
        this.list.addAll(list);
    }
}

Answer 2

List<? extends Super> List<? extends Super> means the list can consume any unknowns as long is it is a Super class of a child of Super .

Unfortunately, you cannot add an unknown (as the compiler wouldn't match the type of element through some "capture of" rules) to a list, except null.

The best principle is to use the PECS principle ( P roducer E xtends, C onsumer S uper). Seeing your list is a consumer, you should have a List<? super Super> List<? super Super> to consumer unknowns.

Answer 3

I infer you want the class Sub to extend class Super as:

class Super {}
class Sub extends Super {}

Even if you write the classes as above, the wildcard prevents you from adding anything to a list, since it serves only for read-only lists. You can probably do your task with the class Wrapper as:

public class Wrapper<T extends Super> {

    private List<T> list = new ArrayList<T>();

    void doStuff(){
        List<Sub> al = new ArrayList<Sub>();
        for(Sub s : al)
            list.add((T) s);
    }

}