List<? extends Number> numbers = new ArrayList<Double>() {{ add(2.5); }};
Number n = numbers.get(0);

List<? super Number> numbers = new ArrayList<Number>() {{ add(2.5); add(100f); }}
numbers.add((short) 256);

public static <T> void findMinMax(Stream<? extends T> stream) {
    Stream<T> sortedStream = stream.sorted(comparator);
}