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Flushing a stream ensures that all data that has been written to that stream is output, including clearing any that may have been buffered. Some streams are buffered to aid performance, e.g. a stream writing to disk may buffer until the content reaches a block size.
The flush() method in Python file handling clears the internal buffer of the file. In Python, files are automatically flushed while closing them. However, a programmer can flush a file before closing it by using the flush() method.
flush() writes the content of the buffer to the destination and makes the buffer empty for further data to store but it does not closes the stream permanently. That means you can still write some more data to the stream. But close() closes the stream permanently.
From the docs of the flush method:
Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.
The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.
When you write data to a stream, it is not written immediately, and it is buffered. So use flush() when you need to be sure that all your data from buffer is written.
We need to be sure that all the writes are completed before we close the stream, and that is why flush() is called in file/buffered writer's close().
But if you have a requirement that all your writes be saved anytime before you close the stream, use flush().
When we give any command, the streams of that command are stored in the memory location called buffer(a temporary memory location) in our computer. When all the temporary memory location is full then we use flush(), which flushes all the streams of data and executes them completely and gives a new space to new streams in buffer temporary location. -Hope you will understand
If the buffer is full, all strings that is buffered on it, they will be saved onto the disk. Buffers is used for avoiding from Big Deals! and overhead.
In BufferedWriter class that is placed in java libs, there is a one line like:
private static int defaultCharBufferSize = 8192;
If you do want to send data before the buffer is full, you do have control. Just Flush It. Calls to writer.flush() say, "send whatever's in the buffer, now!
reference book: https://www.amazon.com/Head-First-Java-Kathy-Sierra/dp/0596009208
pages:453
Streams are often accessed by threads that periodically empty their content and, for example, display it on the screen, send it to a socket or write it to a file. This is done for performance reasons. Flushing an output stream means that you want to stop, wait for the content of the stream to be completely transferred to its destination, and then resume execution with the stream empty and the content sent.
For performance issue, first data is to be written into Buffer. When buffer get full then data is written to output (File,console etc.). When buffer is partially filled and you want to send it to output(file,console) then you need to call flush() method manually in order to write partially filled buffer to output(file,console).
In addition to other good answers here, this explanation made things crystal clear for me:
A buffer is a portion in memory that is used to store a stream of data (characters). These characters sometimes will only get sent to an output device (e.g. monitor) when the buffer is full or meets a certain number of characters. This can cause your system to lag if you just have a few characters to send to an output device. The flush() method will immediately flush the contents of the buffer to the output stream.
Source: https://www.youtube.com/watch?v=MjK3dZTc0Lg
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