Java – how to return type the return type is also the upper and lower limits of method parameters

Let's assume 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 problem is about public static < T > t testme (list LIST1, list List2) If there are three classes, how does the compiler determine the T type: A, B, C,? When I analyze collections This problem occurs when copying

Solution

In all three cases, the compiler infers the type C of the type parameter t

Most specific type meets the constraint

For the first two statements,

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

Both B and C match because list < b > matches list and list < C > match list and list < b > match list and list < C > match list The compiler selects the most specific type to match. C. you can compile with explicit type parameters to enable 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, which is what the compiler chooses for t

C c = testMe(listB,listC);

This happens because the assigned variable is of type C, and B cannot be assigned to C