FileReader or FileWriter object flush method invokation in java

I 开发者_运维知识库read that invoking flush method guarantees that the last of the data you thought you had already written actually gets out to the file.I didn't get the meaning of this statement can any one explain clearly what actually flush method invocation will do?

The writers are usually buffered so it waits for the buffer to be filled before it writes it to the file. Flush tells to write the buffer even though it might not be filled yet. It's usually useful when you finish the writing since the last buffer may not be full but you want to finish the writing.

Many streams have internal buffers which they use to store data before it is passed on. This prevents a file stream from having to continually write each individual byte to disk (which can be quite expensive). The flush command forces a stream to clear its internal buffers so that, in this case, everything is forced to disk.