Using flush() before close()

I guess in many cases it's because they don't know close() also invokes flush(), so they want to be safe.

Anyway, using a buffered stream should make manual flushing almost redundant.

Developer get into a habit of calling flush() after writing something which must be sent.

IMHO Using flush() then close() is common when there has just been a write e.g.

// write a message
out.write(buffer, 0, size);
out.flush();

// finished
out.close();

As you can see the flush() is redundant, but means you are following a pattern.