Is there a widely used util/library method in Java to tell if all elements in a collection are unique?

I'm looking for a method that, if it were in the standard library, would probably be something like boolean Collections#areAllUnique(Collection). I want a method that returns true iff there are no duplicates in the collection.

Obviously, this would return true for any (properly implemented) Set.

It's also easy to write yourself, as something like collection.size() == new HashSet(collection).size(). However, there are a few reasons I'd still like to see it as a one-line library method:

1. It's easier
2. The above is not as efficient as it could be; the method could avoid building the entire second set if it detects any duplicates.

A good implementation might look like

public static <T> boolean areAllUnique(Collection<T> collection) {
  if (collection instanceof Set) {
    return true;
  }
  HashSet<T> set = new HashSet<>(collection.size());
  for (T t : collection) {
    if (set.contains(t)) {
      return false;
    }
    set.add(t);
  }
  return true;
}

Is there a method like this somewhere in the Java standard library or any of the well-known libraries out there (Guava, etc.)?

I'm pretty sure that no such method exists in Guava.

Your method is mostly fine; I would make a couple of tweaks to it to make it marginally cleaner/more efficient:

// T doesn't really do anything. A wildcard is sufficient.
public static boolean areAllUnique(Collection<?> collection) {
  if (collection instanceof Set) {
    return true;
  }
  HashSet<Object> set = new HashSet<>(collection.size());
  for (Object t : collection) {
    // add only returns true if the set is changed, so you don't need to
    // do contains/add separately.
    if (!set.add(t)) {
      return false;
    }
  }
  return true;
}

If you want to make the API designers cry (because predicates should be stateless), you can also do the same thing more concisely:

return collection instanceof Set
    || collection.stream().allMatch(new HashSet<>(collection.size())::add);