How to parse and manipulate a binary stream

Apologies in advance if my question is silly or trivial. I'm trying to
write a servlet that reads data from another source in byte[] form and,
having parsed this data stream and made a couple of modifications,
sends the modified data to an appropriate application whereby it can be
rendered in Excel or PDF format.

What I need to find out is how I can parse the incoming data, detect
certain string patterns in the data, and manipulate that information to
generate a new data stream.

TIA,
tc

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