第 32 条：谨慎并用泛型和可变参数

谨慎并用泛型和可变参数.jpeg

static void dangerous(List<String>... stringLists) {
    List<Integer> intList = List.of(42);
    Object[] objects = stringLists;
    objects[0] = intList; // Heap pollution
    String s = stringLists[0].get(0); // ClassCastException
}

static <T> T[] toArray(T... args) {
    return args;
}
static <T> T[] pickTwo(T a, T b, T c) {
    switch(ThreadLocalRandom.current().nextInt(3)) {
        case 0: return toArray(a, b);
        case 1: return toArray(a, c);
        case 2: return toArray(b, c);
    }
    throw new AssertionError(); // Can't get here
}

@SafeVarargs
static <T> List<T> flatten(List<? extends T>... lists) {
    List<T> result = new ArrayList<>();
    for (List<? extends T> list : lists)
        result.addAll(list);
    return result;
}