Streams

Input and output are usually performed with streams, which are sequences of bytes. In input operations, the bytes flow from a device (e.g., a keyboard, a disk drive, a network connection) to main memory. In output operations, bytes flow from main memory to a device (e.g., a display screen, a printer, a disk drive, a network connection, etc.).

When program execution begins, three streams are connected to the program automatically. Normally, the standard input stream is connected to the keyboard, and the standard output stream is connected to the screen. A third stream, the standard error stream (System.err), is typically connected to the screen and is used to output error messages to the screen so they can be viewed immediatelyeven when the standard output stream is writing into a file. Operating systems typically allow these streams to be redirected to other devices. Streams are discussed in detail in Chapter 14, Files and Streams, and Chapter 24, Networking.