Flush the stream. If the stream has saved any characters from the various write() methods in a buffer, write them
immediately to their intended destination. Then, if that destination is another character or byte stream, flush
it. Thus one flush() invocation will flush all the buffers in a chain of Writers and OutputStreams.
If the intended destination of this stream is an abstraction provided by the underlying operating system, for
example a file, then flushing the stream guarantees only that bytes previously written to the stream are passed to
the operating system for writing; it does not guarantee that they are actually written to a physical device such as
a disk drive.
Close the stream, flushing it first. Once a stream has been closed, further write() or flush() invocations will
cause an IOException to be thrown. Closing a previously-closed stream, however, has no effect.