java - What's the meaning of the '&' character in the returned value?

Question

public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
        Function<? super T, ? extends U> keyExtractor)
{
    Objects.requireNonNull(keyExtractor);
    return (Comparator<T> & Serializable)
        (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
}

The part I'm interested in is this: return (Comparator<T> & Serializable)

Thanks for your help!

Answer 1

This means that the resulting value will be cast to Comparator and Serializable (i.e. a serializable comparator)

Note that when doing casts like this one, you're allowed to specify only one class (and infinite amount of interfaces), because it's not possible for a class to inherit from more than one super-class.