Java Stream: find an element with a min/max value of an attribute

Question

I have a stream of objects and I would like to find the one with a maximal value of some attribute that\'s expensive to calculate.

As a specific simple example, say

Answer 1

Here's a variant using an Object[] as a tuple, not the prettiest code but concise

String coolestString = stringList
        .stream()
        .map(s -> new Object[] {s, coolnessIndex(s)})
        .max(Comparator.comparingInt(a -> (int)a[1]))
        .map(a -> (String)a[0])
        .orElse(null);

Answer 2

How about using two streams, one to create a map with the pre-calculated values and a second using the map's entry set to find the max value:

        String coolestString = stringList
            .stream()
            .collect(Collectors.toMap(Function.identity(), Test::coolnessIndex))
            .entrySet()
            .stream()
            .max((s1, s2) -> Integer.compare(s1.getValue(), s2.getValue()))
            .orElse(null)
            .getKey();

Answer 3

just create your (object,metric) pairs first:

public static <T> Optional<T> maximizeOver(List<T> ts, Function<T,Integer> f) {
    return ts.stream().map(t -> Pair.pair(t, f.apply(t)))
        .max((p1,p2) -> Integer.compare(p1.second(), p2.second()))
        .map(Pair::first);
}

(those are com.googlecode.totallylazy.Pair's)