Java Generics not able to add child to List of parent

Question

Why the below code is showing a compilation error. Doesn't extend mean it can take any subclass of Number, Also compilation is shown in the add method not while defining the List if it doesn't support this construct it should show compilation error while declaring.

List<? extends Number> list = new ArrayList<Integer>();
list.add(1)

This wild card allows you can assign a collection of type `Number` or its subtype. That's why `List extends Number> list = new ArrayList();` compiles correctly. — Alexander Ivanchenko, Feb 12 '22 at 18:28
But you cant add anything such a list `List extends Number>` because this variable can be assigned to a `List` or `List` which are not compatible with Integer. This list is `read-only` — Alexander Ivanchenko, Feb 12 '22 at 18:31
so basically there's no use of such direct instantiation unless used as method arguments. — garg10may, Feb 12 '22 at 18:33
Exactly, a wild-card-collection will be useless if it'll be assigned with a newly created empty collection. But it's powerful as an argument of a method. — Alexander Ivanchenko, Feb 12 '22 at 18:39
https://stackoverflow.com/questions/933447/how-do-you-cast-a-list-of-supertypes-to-a-list-of-subtypes pls check the answer by @Salandur, so misleading, correct me if I am wrong — garg10may, Feb 12 '22 at 18:39
the reference you've provided is related to casting will generics — Alexander Ivanchenko, Feb 12 '22 at 19:51
check out this link https://stackoverflow.com/questions/16707340/when-to-use-wildcards-in-java-generics — Alexander Ivanchenko, Feb 12 '22 at 20:02
If you see the answer by @R-tooR using `super` it works, I am more confused now :P — garg10may, Feb 13 '22 at 06:38

score 0 · Answer 1 · answered Feb 12 '22 at 19:11

Number literals are of type int. Using construction <? extends Number> you ensure, that you can pass only instances of Number class, or any classes, that Number inherits from (that is java.lang.Object). If you want to add subtypes of Number class, e.g. Integers you should use <? super Number> (it means: for whose Number is a super class) You may want to use:

List<? super Number> list = new ArrayList<>();
list.add(1); //no compile-time error

However, if you want to use elements of the collection, you may use them as instances of classes that extends Number class.

    List<? super Number> list = new ArrayList<>();
    list.add(1);
    list.add(2);
    print(list);

    List<Object> objs = new ArrayList<>();
    objs.add(1);
    print(objs);

    List<Number> nums = new ArrayList<>();
    nums.add(1);
    print(nums);
}

public static void print(List<? super Number> list){ // you can pass List of instances of type Number or upper
    for(Object num: list){ // but you have to refer to them as Objects (it is an upper class of Number
        System.out.println(num);
    }
}

Java Generics not able to add child to List of parent

1 Answers1