Many output streams buffer writes to improve performance. Rather than sending each byte to its destination as it’s written, the bytes are accumulated in a memory buffer ranging in size from several bytes to several thousand bytes. When the buffer fills up, all the data is sent at once. The flush() method forces the data to be written whether or not the buffer is full:

public void flush() throws IOException

This is not the same as any buffering performed by the operating system or the hardware. These buffers will not be emptied by a call to flush(). (Then sync() method in the FileDescriptor class, discussed in Chapter 12, can sometimes be used to empty these buffers.) For example, assuming out is an OutputStream of some sort, you would call out.flush() to empty the buffers.

If you only use a stream for a short time, you don’t need to flush it explicitly. It should be flushed when the stream is closed. This should happen when the program exits or when you explicitly invoke the close() method:

public void close() throws IOException

For example, again assuming out is an OutputStream of some sort, calling out.close() closes the stream and implicitly flushes it. Once you have closed an output stream, you can no longer write to it. Attempting to do so will throw an IOException.

You only need to flush an output stream explicitly if you want to make sure data is sent before you’re through with the stream. For example, a program that sends a burst of data across the network periodically should flush after each burst of data is written to the stream.

Flushing is often important when you’re trying to debug a crashing program. All streams flush automatically when their buffers fill up, and all streams should be flushed when a program terminates normally. If a program terminates abnormally, however, buffers may not get flushed. In this case, unless there is an explicit call to flush() after each write, you can’t be sure the data that appears in the output indicates the point at which the program crashed. In fact, the program may have continued to run for some time past that point before it crashed.

System.out, System.err, and some (but not all) other print streams automatically flush after each call to println() and after each time a new line character (' ') appears in the string being written. Whether auto-flushing is enabled can be set in the PrintStream constructor.