Know it all #1:  You must concatenate StringBuffer, not String in order to
achieve best performance.  Convert to String when finished.
Know it all #2:  No you don't.  The JIT optimizes concatenation for you.

All strings are 256 in length and assembled one character at a time.
Concatenating 512 Strings...
Concatenating 512 buffers and turning each into a String...
StringBuffer concat took 56ms
String concat took 23488ms
Know it all #1 was correct.
Build using a buffer first, then convert to a String.