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

Stream<String> stringStream = stringList.stream();
String coolest = stringStream.reduce((a,b)-> 
    coolnessIndex(a) > coolnessIndex(b) ? a:b;
).get()

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);