ArrayList用不同的数据类型初始化

Question

Firstly, why does the first line compile while the second does not? Secondly, in the case of the second line, do both types always need to be the same ie Integer on the left and Integer on the right. Or is it possible to have different types on the left and the right?

    List<? super Integer> nums1 = new ArrayList<Number>();  //COMPILES
    List<Integer> nums2 = new ArrayList<Number>();          //DOES NOT COMPILE

Answer 1

Your question is esentially a duplicate of this SO-article but in short:

? super T ? super T means "anything that is a superclass of T". Number is a superclass of Integer so this is accepted. The second line doesn't work simply because Number is not Integer . It wouldn't work vice versa, either so

ArrayList<Number> nums2 = new ArrayList<Integer>();

leads to a compile error as well. For that you can use

ArrayList<? extends Number> nums2 = new ArrayList<Integer>();