Why is it necessary to flush the output buffer when it was just created?

Question

In the following scenario

ObjectOutputStream output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
// Do stuff with it

Why is it always necessary to flush the buffer after initial creation?
I see this all the time and I don't really understand what has to be flushed. I kind of expect newly created variables to be empty unless otherwise is specified.

Kind of like buying a trash-can and finding a tiny pile of trash inside that came with it.

Answer 1

In over 15 years of writing Java on a professional level I've never once encountered a need to flush a stream before writing to it.
The flush operation would do nothing at all, as there's nothing to flush.
You want to flush the stream before closing it, though the close operation should do that for you it is often considered best practice to do it explicitly (and I have encountered situations where that did make a difference, where apparently the close operation did not actually do a flush first.
Maybe you are confused with that?