java - Output stream over a StringBuilder

I think this has been answered but I can't seem to find
it.

I have an instance method which writes some contents to
an output
stream

writeTo(OutputStream){

            //class specific
            logic
}

I want it to
get these contents into a StringBuilder. I can do this via a temporary file but that
does not seem right. I want to do something
like:

Stringbuilder sb = /*
            */;
OutputStream os = outForStringBuilder(sb);//not sure how to do
            this
instance.writeTo(os); //This should write the contents to
            Stringbuilder