How to merge multiple (arbitrary number of) Set<String> into one in Java?

Question

I have multiple Set<String> that I need to merge into one Set<String>. How do I do this operation in Java? Note, I am using using the guava API as best as I can to help out. For example, I have 3 classes as follows.

public class One {
 public static Set<String> SET = Sets.newHashSet("a","b","c");
}
public class Two {
 public static Set<String> SET = Sets.newHashSet("a","d","e","f");
}
public class Three {
 public static Set<String> SET = Sets.newHashSet("w","x","y","f");
}

Now, I need to merge any combination of these sets into one. For, example, I may need to merge

• One.SET + Two.SET + Three.SET into one to produce { "a","b","c","d","e","f","w","x","y" },
• One.SET + Three.SET into one to produce { "a","b","c","w","x","y","f" },
• Two.SET + Three.SET into one to produce { "a","d","e","f","w","x","y" },
• and so on

I created a method to merge an array of sets, Set<String>[], but that doesn't work (explained here by this SO post Creating an array of Sets in Java). Here's the code to merge. It works (compiles).

public static Set<String> immutableSetOf(Set<String>[] sets) {
 Set<String> set = new HashSet<String>();
 for(Set<String> s : sets) {
  set.addAll(s);
 }
 return ImmutableSet.copyOf(set);
}

Here's the calling code; it doesn't work (doesn't compile).

Set<String> set = Utils.immutableSetOf(new Set<String>[] { One.SET, Two.SET });

So, I modified my merging method to operate on List<Set<String>> instead of Set<String>[]. Only the argument type changed, but I put it here for completeness.

public static Set<String> immutableSetOf(List<Set<String>> sets) {
 Set<String> set = new HashSet<String>();
 for(Set<String> s : sets) {
  set.addAll(s);
 }
 return ImmutableSet.copyOf(set);
}

So, now my calling code looks like the following.

Set<String> set = Utils.immutableSetOf(
 Lists.newArrayList(
  One.SET, Two.SET));

This code does not compile, since Lists.newArrayList(...) is returning Set<String> and not List<Set<String>>. The method Lists.newArrayList(...) is overloaded, and the signature of the method that is used when I pass in sets is, List.newArrayList(Iterable<? extends E> elements).

So, the question is, how do I define a method to merge an arbitrary number of Set<String> while considering the calling code? I note that the compilation problems are on the calling code (not the merging method), but perhaps the solution also relates to the merging code?

Update: I also tried varargs but that produces its own warning (Is it possible to solve the "A generic array of T is created for a varargs parameter" compiler warning?). The merging method signature is now the following.

public static Set<String> immutableSetOf(Set<String>... sets)

The calling code is now the following, and I get "Type safety: A generic array of Set is created for a varargs parameter".

Set<String> set = Utils.immutableSetOf(One.SET, Two.SET);

Update: For the accepted answer, I did the following.

@SuppressWarnings("unchecked")
Set<String> set = Utils.immutableSetOf(Set[] { One.SET, Two.SET });

Answer 1

Recommend com.google.common.collect.Sets#union(set1, set2) to get the merge instead of Set.addAll under hood, since Guava is already in your dependencies.

Reason: it's view which is memory effective, and also unmodifiable.

plus: I should have post it as a comment, sorry.