FastCat is a streamlined speedy version of StringBuffer/StringBuilder that
is more efficient because it does not allocate a buffer until the very
last minute, and allocates it the exact size needed.  You use it in
writing your own Java Programs.  It works very similarly to StringBuilder,
so the Javadoc should be all you need.
FastCat is null-safe.  If you pass a null or a 0-length string to one of
the .append methods it does nothing.  This means you can streamline your
application code.  You don't have to explicitly avoid appending nulls, an
error that can slip through testing with StringBuilder since it may occur
only rarely.  FastCat does not work well if you append one character at a
time to your FastCat.  For that sort of application, use the traditional
StringBuilder.  FastCat works very quickly with minimal RAM usage if you
can accurately predict roughly how many chunks you will append but have
only a vague idea of the final total size.  By minimising RAM usage, your
garbage collection occurs less frequently, which speeds it up.  I got a
10% speedup when I switched over the HTMLMacros app to use it.
The basic idea of FastCat is you estimate how many chunks you concatenate,
not the length of the output.