latestVersion.java

/*
    Given a list of Things, return a map of key -> value where value is the 
    value of the highest version of the Thing with that key.
*/

public interface Thing {
    String getKey();
    String getValue();
    int getVersion();
}

Collection<Thing> things = ...
Thing identityThing = ...
    Map<String, String> latest = things.stream()
        .collect(Collectors.groupingBy(Thing::getKey, Collectors.reducing(identityThing, (t1, t2) -> (t1.getVersion() > t2.getVersion()) ? t1 : t2)))
        .entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().getValue()))
    ;