PECS (producer extends, consumer super)

Definition

PECS means: if a collection only gives you values, use ? extends T (producer extends). If it only receives values, use ? super T (consumer super). It makes a method accept more types while staying type-safe.

Example code

Copy from a producer into a consumer

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

class Demo {
    // source produces Ts (extends), destination consumes Ts (super)
    static <T> void copy(List<? extends T> source, List<? super T> destination) {
        for (T item : source) destination.add(item);
    }

    public static void main(String[] args) {
        List<Integer> integers = List.of(1, 2, 3);
        List<Number> numbers = new ArrayList<>();
        List<Object> objects = new ArrayList<>();

        copy(integers, numbers);       // Integers can go into a list of Number
        copy(integers, objects);       // and into a list of Object
        System.out.println(numbers);
        System.out.println(objects);
    }
}

// Output:
// [1, 2, 3]
// [1, 2, 3]