Wildcards

Definition

A wildcard ? stands for "some unknown type". ? extends Number means "Number or any subtype", which is good for reading. ? alone means any type at all.

Example code

Summing a list of any number type

import java.util.List;

class Demo {
    static double sum(List<? extends Number> numbers) {
        double total = 0;
        for (Number n : numbers) total += n.doubleValue();
        return total;
    }

    static void printAll(List<?> items) {            // any kind of list
        for (Object item : items) System.out.print(item + " ");
        System.out.println();
    }

    public static void main(String[] args) {
        System.out.println(sum(List.of(1, 2, 3)));            // List<Integer>
        System.out.println(sum(List.of(1.5, 2.5)));           // List<Double>
        printAll(List.of("a", "b"));
        printAll(List.of(1, 2));
    }
}

// Output:
// 6.0
// 4.0
// a b 
// 1 2