Java Generics Method header

Question

Say i have some method header..

public static <E> void myMethodName(E[], Comparator<E> comparator).

Why would I get a type error when attempting to call:

myMethodName(int[], 
    new Comparator<Integer>() {
        public int compare(Integer e1, Integer e2) { return e1 - e2; }
    }
);

any ideas?

Answer 1

First, let's look at a few things.

• You have a generic method (it's missing a variable though, but I'll overlook that) which requires a generic type. Java infers, based on the fact that your array is of type int , that the method is type bound to Integer .

• You have a primitive int[] , which you are providing as the first argument. In the formal signature, it is type bound to Integer .

The problem here: an int[] is not equivalent to an Integer[] . They are two completely different objects (since they're both arrays as opposed to objects that can be autoboxed).

The proper way to avoid the issue is to use an Integer[] instead. The elements that you place inside the array will be autoboxed according to the rules of autoboxing and conversion, though.

Answer 2

尽管自动装箱可能会使问题感到困惑，但int和Integer不是同一类型。

Answer 3

@user2574476 : you can point E to any reference type not primitive type. So instead of using int[], you should use Integer[].