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.  