write to file one line at a time and keep all contents

[stringargs]

So, say I have a text file like this:

aaaaaaaaaaa
bbbbbbbbbbb
ccccccccccc

I need to replace the first line with "cccc" and I need that done without copying the whole file, its contents or making a new file and renaming it later.
Ideas?

[NRitH]

It's easy enough to avoid renaming it (just open it, read it, close it, change it, open it again, write it, and close it again), but what's wrong with copying its entire contents? It's easier to do that with a BufferedReader & BufferedWriter than using something like a RandomAccessFile, which is a pain in the ass, and is better suited to binary files than to plain text ones.