[读书笔记]about Stream

 

The main reason for using a stream as the basis for input and output operations is to make your program　code for these operations independent of the device involved. This has two advantages. First, you　don’t have to worry about the detailed mechanics of each device, which are taken care of behind the　scenes. Second, your program will work for a variety of input/output devices without any changes to　the code.

 

Stream input and output methods generally permit very small amounts of data, such as a single character　or byte, to be written or read in a single operation. Transferring data to or from a stream like this may　be extremely inefficient, so a stream is often equipped with a buffer in memory, in which case it is called　a buffered stream. A buffer is simply a block of memory that is used to batch up the data that is transferred　to or from an external device. Reading or writing a stream in reasonably large chunks will reduce　the number of input/output operations necessary and thus make the process more efficient.

 

When you write to a buffered output stream, the data is sent to the buffer and not to the external device.　The amount of data in the buffer is tracked automatically, and the data is usually sent to the device when　the buffer is full. However, you will sometimes want the data in the buffer to be sent to the device before　the buffer is full, and methods are provided to do this. This operation is usually termed flushing the　buffer.

 

Buffered input streams work in a similar way. Any read operation on a buffered input stream will read　data from the buffer. A read operation for the device that is the source of data for the stream will be read　only when the buffer is empty and the program has requested data. When this occurs, a complete buffer　full　of data will be read automatically from the device, if sufficient data is available.