Comparator.comparing-da Serializable-ga uzatish [duplikat]

java.util.Comparator JDK 8 manba kodida, men qiziqarli kodni topdim:

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

Olingan lambdani nima uchun Comparator<T> ga quyayotganimiz juda aniq, lekin uni Serializable ga quyishdan maqsad nima?