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<Integer>) that potentially contains many elements. I need a Collection that contains the elements from that set plus one additional element. I have kludgy code in place to copy the set, then append the element, but I'm looking for The Right Way that keeps things as efficient as possible.

I have Guava available, though I do not require its use.

Answer 1

Not sure about performance, but you can use Guava's ImmutableSet.Builder:

import com.google.common.collect.ImmutableSet  // ... Set<Integer> newSet = new ImmutableSet.Builder<Integer>()                                 .addAll(oldSet)                                 .add(3)                                 .build(); 

Of course you can also write yourself a helper method for that:

public static <T> Set<T> setWith(Set<T> old, T item) {   return new ImmutableSet.Builder<T>().addAll(old).add(item).build(); }  // ... Set<Integer> newSet = setWith(oldSet, 3); 

Answer 2

Using Java 8 you can also use streams for that effect

Stream.concat(oldSet.stream(),               Stream.of(singleElement))       .collect(Collectors.toSet()) 

By the way since JDK 10, the Collectors also allow to accumulate to immutable types (the same as the ones created by the static factories like Set.of()) :

Stream.concat(oldSet.stream(),               Stream.of(singleElement))       .collect(Collectors.toUnmodifiableSet())