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.
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.
Java 8 introduced the method
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.