The + operator uses public String concat(String str) internally. This method copies the characters of the two strings, so it has memory requirements and runtime complexity proportional to the length of the two strings. StringBuilder works more efficent.
However I have read here that the concatination code using the + operater is changed to StringBuilder on post Java 4 compilers. So this might not be an issue at all. (Though I would really check this statement if I depend on it in my code!)
