Factually incorrect. Creating new strings by concatenation requires reallocation because Strings are immutable in java. Using a Builder is over 700x faster:

Concat code: http://pastebin.com/CcezmJbf

Builder code: http://pastebin.com/Dd6kzcKa

Results, using JRE 7:

> time java concat data.blk Finished. java foo data.blk 170.08s user 1.52s system 102% cpu 2:48.12 total

> time java builder data.blk Finished. java builder data.blk 0.30s user 0.05s system 151% cpu 0.230 total