Java Generics Wildcards Explained: PECS (Producer Extends, Consumer Super)

One rule that tells you whether to write ? extends or ? super

Generics make your code type-safe, but wildcards (? extends, ? super) are where most people freeze. The syntax looks random, and picking the wrong one gives confusing compiler errors.

You don’t need to memorize the theory. You need one rule: PECS.

Start Here, Generics Are Strict

List<Integer> ints = new ArrayList<>();
List<Number> nums = ints;   // compile error

Integer is a Number, but List<Integer> is not a List<Number>.

The compiler blocks this for a good reason. If it were allowed, this would break:

nums.add(3.14);             // a Double inside a list of Integers

Remember: generics are invariant. List<Integer> and List<Number> are unrelated types.

Wildcards are the way to relax this rule safely.

Generic Methods

Put <T> before the return type. T is decided by the caller.

static <T> T firstOrNull(List<T> list) {
    return list.isEmpty() ? null : list.get(0);
}

String s  = firstOrNull(List.of("a", "b"));   // T = String
Integer i = firstOrNull(List.of(1, 2));       // T = Integer