Opposite of StringSplit

A combination of StringJoin and Riffle:

res = StringSplit["a b c d e f g"," "];
StringJoin@Riffle[res," "]

In version 10.1 you can use StringRiffle:

res = StringSplit["a b c d e f g", " "];
StringRiffle[res]

Use no second argument for spaces, or something else for something else. A nice advantage of StringRiffle is that res elements can be non string elements, and it will be automatically converted. It's something I miss in StringJoin.

PS: this answer is based on Docs, I don't have V10.1.

Take care with ToString@Row in V9 or older. See this post, about some problems.

Sometimes you can you this alternative:

res = StringSplit["a b c d e f g", " "];

ToString @ Row[res, " "]