What is an efficient and elegant way to add a single element to an immutable set?

Question

I have an immutable set (cast as a Set) that potentially contains many elements. I need a Collection that contains the elements from that set plu

Answer 1

You might consider Sets.union(). Construction would be faster, but use slower.

public static  Set setWith(Set old, T item) {
  return Sets.union(old, Collections.singleton(item);
}

(com.google.common.collect.Sets & java.util.Collections)