How can I collect only the elements of the greatest length with Java Streams?

Try this:

List<String> strings = Arrays.asList("long word", "short", "long wwww", "llll wwww", "shr");

List<String> longest = strings.stream()
        .collect(groupingBy(String::length, TreeMap::new, toList()))
        .lastEntry()
        .getValue();

System.out.println(longest);

Output:

[long word, long wwww, llll wwww]

Well, I don't know if this will be more elegant but it should do what you want:

List<String> strings = Arrays.asList("long word", "short", "long wwww", "llll wwww", "shr");

List<String> longest = strings.stream()
        .collect(Collectors.groupingBy(String::length))     // Build Map<Length, List<Strings>>
        .entrySet().stream()                                // EntrySet stream of said map
        .max(Map.Entry.comparingByKey())                    // Keep max length
        .map(Map.Entry::getValue)                           // Get value of max length
        .orElse(Collections.emptyList());                   // Or return an empty list if there's none

System.out.println(longest);

Output:

[long word, long wwww, llll wwww]

You may consider it uglier, but a custom collector is definitely correct, more efficient, and even parallelizes nicely:

Collector<String, List<String>, List<String>> collector = Collector.of(
   ArrayList::new,
   (list, elem) -> {
     if (list.isEmpty() || elem.length() == list.get(0).length()) {
       list.add(elem);
     } else if (elem.length() > list.get(0).length()) {
       list.clear();
       list.add(elem);
     }
   },
   (list1, list2) -> {
     int len1 = list1.isEmpty() ? -1 : list1.get(0).length();
     int len2 = list2.isEmpty() ? -1 : list2.get(0).length();
     if (len1 < len2) {
       return list2;
     } else if (len1 > len2) {
       return list1;
     } else {
       list1.addAll(list2);
       return list1;
     }
   });

return strings.stream().collect(collector);