Bounded Type Parameters

Bounded type parameters

A bare <T> promises almost nothing. A bound makes the promise concrete:

// T can be any Number subtype — so .doubleValue() is legal on it
static double average(List<? extends Number> xs) { ... }

// <T extends Comparable<T>> lets you call compareTo on values of type T
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;
}

Two bound forms:

• Type-parameter bound <T extends Comparable<T>> — used when T must flow through the method (in and out).
• Wildcard bound List<? extends Number> — used when T only flows out of the list (you read it, never write).

Multiple bounds: <T extends Number & Comparable<T>> — class first, interfaces after.