D:

ffs.java

CompletableFuture<Map<UUID, Collection<SuperCow>> method() {
  CompletableFuture<Collection<SuperCow>> starter = callProvider();
  return starter
    .thenApply(c -> c.stream().collect(Collectors.groupingBy(SuperCow::getGroupingId))) // Map<UUID, List<SuperCow>>
    .thenApply(m -> m.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))); // FFUUUUUUUUUU
}