what's the purpose of BufferedOutputStream?

Question

I am wondering the purpose of BufferedOutputStream, performance gain when using it?

Answer 1

Here is the line from API of BufferedOutputStream:

The class implements a buffered output stream. By setting up such an output stream, an application can write bytes to the underlying output stream without necessarily causing a call to the underlying system for each byte written.

It can do most of operations within the buffer, and without a call to the underlying system.

For example, consider writing to file: without buffer, it has to make a system call for every single byte, which is obviously slow.

Answer 2

As its name suggests, BufferOutputStream has an internal buffer (byte[]) to which contents of individual small writes are first copied. They are written to the underlying OutputStream when buffer is full, or the stream is flushed, or the stream is closed. This can make a big difference if there is a (relatively large) fixed overhead for each write operation to the underlying OutputStream, as is the case for FileOutputStream (which must make an operating system call) and many compressed streams.

At the same time, many stream-based libraries use their own buffering (like XML and JSON writers), and use of BufferedOutputStream provides no benefit. But its own overhead is relatively low so there isn't much risk.