Generics error in java

Question

ArrayList<Integer> arr = new ArrayList<>();
ArrayList<Double> doub = new ArrayList<>();
        arr.add(5);
        arr.add(6);
        arr.add(7);
        doub.add(6.5);
        doub.add(8.5);
        doub.add(7.5);
        calc_test(arr);
        calc_test(doub);

public static void calc_test(List<Number> list) {
        for (Number obj : list) {
            System.out.println(obj + "");
        }
    }

I know this is an error in java and can be corrected by adding ? extends Number in the function parameter, but why does the normal way not work? After all double and integer are a type of number and a list of those types should still be a type of number, so why is that an error?

Edit into your question the details of the error. – peterh Sep 23 '15 at 12:52 — peterh, Sep 23 '15 at 12:52

score 1 · Answer 1 · answered Sep 23 '15 at 12:26

1

Double and Integer is subclass of Number but ArrayList<Double> and ArrayList<Integer> are not subclass of ArrayList<Number>. For more detail here

answered Sep 23 '15 at 12:26

Vaseph

704
1
8
20

I have read the java docs and i needed a simpler explanation as to why such a thing was not possible. – Jude Fernandes Sep 23 '15 at 12:32

Generics error in java

1 Answers1