Generics

Generics move type mistakes from runtime to compilation. Type parameters appear on classes, interfaces, and methods; bounds state required capabilities.

Bounds and variance

static double total(List<? extends Number> values) {
    double sum = 0;
    for (Number value : values) sum += value.doubleValue();
    return sum;
}

static void addDefaults(List<? super Integer> target) {
    target.add(0);
}

Generic types are invariant: List<Integer> is not List<Number>. Follow PECS—producer extends, consumer super. Java implements most generics through type erasure, so type arguments are usually unavailable at runtime, primitives cannot be direct arguments, and creating new T() or new T[] is restricted. Avoid raw types and unchecked casts; prefer bounded APIs that express the operations truly needed.