Converting from HashSet<String> to String[]

set.toArray(new String[set.size()]);

Answer of JB Nizet is correct, but in case you did this to transform to a CSV like string, with Java 8 you can now do:

Set<String> mySet = new HashSet<>(Arrays.asList("a", "b", "c"));
System.out.println(String.join(", ", mySet));

Output is: a, b, c

This allows to bypass array notation (the []).