How is type inferred where return type is also upper and lower bound for method parameters

Question

Lets suppose we have the following code:

class A {}

class B extends A {}

class C extends B {}

public static <T> T testMe(List<? super T> list1,List<? extends T> list2) {
    return null;
}

public static void main(String[] args) {
    List<B> listB = new ArrayList<>();
    List<C> listC = new ArrayList<>();
    // All three variants are possible:
    A a=testMe(listB, listC);
    B b=testMe(listB, listC);
    C c=testMe(listB, listC);
}

The question is about public static <T> T testMe(List<? super T> list1,List<? extends T> list2). How does the compiler determine the T type if if have three classes:A,B,C,? This question arose when I analysed Collections.copy.

Answer 1

The compiler is inferring the type C for the type parameter T in all 3 cases.

It is the most specific type that fits the constraints.

[T]he inference algorithm tries to find the most specific type that works with all of the arguments.

For the first 2 statements,

A a = testMe(listB, listC);
B b = testMe(listB, listC);

Both B and C match, because a List<B> matches List<? super B> and List<C> matches List<? extends B>, and a List<B> matches List<? super C> and List<C> matches List<? extends C>. The compiler chooses the most specific type that matches, C.

You could get this to compile with an explicit type parameter, to get the compiler to resolve it to B:

A a = Super.<B>testMe(listB, listC);
B b = Super.<B>testMe(listB, listC);

In the third line, only C matches, so that is what the compiler chooses for T.

C c = testMe(listB, listC);

This happens because the variable being assigned is of type C, and a B can't be assigned to C.