Misunderstanding of flush () method of OutputStream class

Recently used java Of FileOutputStream Writing documents , Call to flush() Method .
So I looked at it FileInputStream Source code of class , Find out flush() In fact, it inherits from its parent class OutputStream Of .
and OutputStream Class flush() But I didn't do anything , See light suddenly , It's “ Look at the truth of the source code ”.
Actually flush() yes Flushable Interface method , The comment on this method in the official document is “Flushes this output stream and forces any buffered output bytes to be written out.”.
OutputStream Method implemented Flushable Interface , And did nothing , It's really confusing , So there was my misunderstanding .

So when flush() It works ？
The answer is ： When OutputStream yes BufferedOutputStream when .

When writing files, you need flush() The effect of , need
FileOutputStream fos = new FileOutputStream("c:\a.txt");
BufferedOutputStream bos = new BufferedOutputStream(fos);
in other words , Need to put FileOutputStream As BufferedOutputStream The arguments of the constructor are passed in , Then on BufferedOutputStream Write operation , To take advantage of the buffer and flush().

see BufferedOutputStream Source code , Find the so-called buffer It's really just a byte[].
BufferedOutputStream Every time write It's actually writing content into byte[], When buffer When the capacity reaches the upper limit , Will trigger a real disk write .
Another way to trigger a disk write is to call flush() 了 .