Bounded wildcards and PECS ("Producer Extends, Consumer Super")
List<? extends Number> — you can read Numbers out of it, but can't add anything (the compiler
doesn't know the exact subtype, so any add could be unsafe). List<? super Integer> — you can write
Integers into it, but reads only guarantee Object. Rule of thumb: if a parameter only produces values for
you to consume, use extends; if it only consumes values you hand it, use super.
java
import java.util.*;
// consumer of Integer -> use `super`
static void addNumbers(List<? super Integer> dst, List<Integer> src) {
for (Integer i : src) dst.add(i);
}
// producer of Number -> use `extends`
static double sumNumbers(List<? extends Number> src) {
double total = 0;
for (Number n : src) total += n.doubleValue();
return total;
}
List<Object> objs = new ArrayList<>();
addNumbers(objs, List.of(1, 2, 3));
System.out.println("PECS 'super' (write Integers into List<Object>): " + objs);
System.out.println("PECS 'extends' (read Numbers from List<Double>): " + sumNumbers(List.of(1.5, 2.5, 3.0)));
System.out.println("same method also accepts List<Integer>: " + sumNumbers(List.of(1, 2, 3)));