Fastest way to put contents of Set to a single String with words separated by a whitespace?

Question

I have a few Sets and want to transform each of these into a single String where each element of the original Set is sep

Answer 1

This can be done by creating a stream out of the set and then combine the elements using a reduce operation as shown below (for more details about Java 8 streams check here):

Optional<String> joinedString = set1.stream().reduce(new 
BinaryOperator<String>() {

     @Override
     public String apply(String t, String u) {

       return t + " " + u;
    }
});
return joinedString.orElse("");

Answer 2

With commons/lang you can do this using StringUtils.join:

String str_1 = StringUtils.join(set_1, " ");

You can't really beat that for brevity.

Update:

Re-reading this answer, I would prefer the other answer regarding Guava's Joiner now. In fact, these days I don't go near apache commons.

Another Update:

Java 8 introduced the method String.join()

String joined = String.join(",", set);

While this isn't as flexible as the Guava version, it's handy when you don't have the Guava library on your classpath.