All the references to Apache Commons are fine (and that is what most people use) but I think the Guava equivalent, JoinerJoiner, has a much nicer API.
You can do the simple join case with
Joiner.on(" and ").join(names)
but also easily deal with nulls:
Joiner.on(" and ").skipNulls().join(names);
or
Joiner.on(" and ").useForNull("[unknown]").join(names);
and (useful enough as far as I'm concerned to use it in preference to commons-lang), the ability to deal with Maps:
Map<String, Integer> ages = .....;
String foo = Joiner.on(", ").withKeyValueSeparator(" is ").join(ages);
// Outputs:
// Bill is 25, Joe is 30, Betty is 35
which is extremely useful for debugging etc.