Generics In Depth

Type erasure

Generics are a compile-time feature. After type checking, the compiler erases type parameters to their bounds (usually Object), so List<String> and List<Integer> share one runtime class. Consequences: no new T[], no instanceof List<String>, and overloads can't differ only by generic type.

Bounded type parameters

<T extends Comparable<T>> restricts T so you can call compareTo. Bounds let a generic method use the capabilities it needs while staying reusable.

static <T extends Comparable<T>> T max(List<T> xs) {
    T best = xs.get(0);
    for (T x : xs) if (x.compareTo(best) > 0) best = x;
    return best;
}

PECS

Producer Extends, Consumer Super. Use ? extends T for a source you only read from, ? super T for a sink you only write to. Collections.copy(List<? super T> dest, List<? extends T> src) is the canonical example.