Java中的下界通配符

Question

Consider the following example,

List<? super Number> list1 = new ArrayList<>();
list1.add(10);
list1.add(10.10);
list1.add(20.20d);

and another list to which i assign list1

List<? super Integer> list = list1;
System.out.println(list.toString());

Now, when i print the list it contains double values also BUT the list is only supposed to hold Integer and anything above Integer .

If the above is fine then shouldn't the following compile as well ?

list.add(30.30);

Answer 1

Everything makes sense if you consider the possible things you could assign to it.

List<? super Number> list1 = new ArrayList<Object>(); // works
List<? super Number> list1 = new ArrayList<Number>(); // works
List<? super Number> list1 = new ArrayList<Integer>(); // doesn't work

Anything that works on the right hand side will accept a Double .

List<? super Integer> list = new ArrayList<Object>(); // works
List<? super Integer> list = new ArrayList<Number>(); // works
List<? super Integer> list = new ArrayList<Integer>(); // works

Anything that you could assign to list1 you could also assign to list . So list = list1 works just fine. But not all of the things you could assign to list will accept a Double , so it doesn't compile.