The Nuts and Bolts of the Java Language

The Standard Input and Output Streams

The character-counting program uses

System.in.read()

to read characters from its input source. The System class is a member of the java.lang package and provides access to system functionality such as standard input and output, copying arrays, getting the current date and time, and properties. The while loop uses the System class's standard input stream to read characters typed in by the user at the keyboard.

In addition, the character-counting program uses the System class's standard output stream to display its output:

System.out.println(. . .);

System.in is a class variable that is a reference to an object implementing the standard input stream. Similarly, System.out is a class variable that is a reference to an object implementing the standard output stream. In addition to System.in and System.out, the System class provides a third stream, System.err, that implements the standard error stream.

The standard I/O streams are a C library concept that has been assimilated into the Java language. Simply put, a stream is a flowing sequence of characters; the standard input stream is a stream that reads characters from the keyboard. The standard input stream is a convenient place for an old-fashioned text-based application to get input from the user. The standard output stream, on the other hand, is a stream that writes its contents to the display. The standard output stream is a convenient place for an old-fashioned text-based application to display its output. And finally, you use the standard error stream to display error messages to the user.

Reading from the Standard Input Stream

The System.in.read() method reads a single character and returns either the character that was read or, if there are no more characters to be read, -1.

As mentioned before, when a program reads from the standard input stream, the program blocks and waits for you to type something in. The program continues to wait for input until you give it some indication that the input is complete. To indicate to any program that reads from the standard input stream that you have finished entering characters, at the beginning of a new line type the end-of-input character appropriate for your system. When the character-counting program receives an end-of-input character the loop terminates and the program displays the number of characters you typed.

Writing to Standard Output

System.out.println() displays its String argument followed by a newline. println() has a companion method print() that displays its argument with no trailing newline.

This page has briefly introduced you to the standard input and output streams provided by the System class. The System class is described completely in Using System Resources. In addition, you can find general information about input and output streams in Input and Output Streams.

The Nuts and Bolts of the Java Language