Casting to Serializable in Comparator.comparing

Question

Looking at java.util.Comparator in the JDK 8 source code, I found interesting code:

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));
    }

It is pretty clear why we are casting the resulting lambda to Comparator<T>, but what is the purpose of casting it to Serializable?

Related: http://stackoverflow.com/questions/22807912/how-to-serialize-a-lambda -- and note how Oracle "strongly discourage" the practice: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#serialization -- where have you found this code? — slim, Mar 22 '17 at 17:05
Interesting. My guess is that unless they do this, nobody could make *any* class serializable if it contained a comparator (or they'd have to put in extra effort). But I've never made much use of `serialize()` so will wait to see a more authoritative answer. — slim, Mar 22 '17 at 17:36
http://stackoverflow.com/questions/41500021/why-does-java-8s-comparator-comparing-cast-the-return-value-to-serializable — Alexis C., Mar 22 '17 at 19:29

score 3 · Answer 1 · answered Mar 22 '17 at 17:26

3

That is making the lambda serializable from what I know.

answered Mar 22 '17 at 17:26

Eugene

117,005
15
201
306

Absolutely right. But what is the purpose? – Andrii Abramov Mar 22 '17 at 19:01
1

@AndriiAbramov I *assume* it has to do with the fact when you serialize some type that *uses* the comparator. Lets say a `TreeSet` or `PriorityQueue`. – Eugene Mar 22 '17 at 19:20
1

@AndriiAbramov, so apperently this was asked before, just found it: http://stackoverflow.com/questions/8642012/why-should-a-comparator-implement-serializable – Eugene Mar 22 '17 at 19:30
Hmm.. Yes. This makes sense. I will close this one as duplicate. Nevertheless, +1! – Andrii Abramov Mar 22 '17 at 19:39
1

@AndriiAbramov you could probably accept it too :) – Eugene Mar 22 '17 at 19:56

Casting to Serializable in Comparator.comparing

1 Answers1