Perhaps you should give us the context of what you are trying to do, as
it isn't trivial to "convert" (as you put it) a BufferedImage object to
an InputStream.

If you are trying to do something such as save the image somewhere, or
upload it somewhere, or read the bytes for some reason, those all would
have a different approach.  You're ultimate goal is more important than
the way you get there.  It's good to say "I'm trying this approach, but
I can't figure out how.  Is there a better way, or if not, how do I get
this way to work."

As for examples, there are plenty on Google.