Java Academy · Lesson

Lower Bounded Wildcards

? super T for consumers.

The Lower Bounded Wildcard

? super T is a lower bounded wildcard. It means "some unknown type that is T or a supertype of T".

List<? super Integer> can refer to a List<Integer>, a List<Number>, or a List<Object>.

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<? super Integer> a = new ArrayList<Number>();
        List<? super Integer> b = new ArrayList<Object>();
        a.add(1);
        b.add(2);
        System.out.println(a + " " + b);
    }
}

Writing to a Consumer

With ? super Integer you can safely add any Integer (or subtype of Integer).

Whatever the real list type, it is at least a list of some Integer supertype, so an Integer always fits.

import java.util.ArrayList;
import java.util.List;

public class Main {
    static void addNumbers(List<? super Integer> dest) {
        for (int i = 1; i <= 3; i++) dest.add(i);
    }
    public static void main(String[] args) {
        List<Number> nums = new ArrayList<>();
        addNumbers(nums);
        System.out.println(nums);
    }
}

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