Standard Streams

All the programming languages provide support for standard I/O where the user's program can take input from a keyboard and then produce an output on the computer screen. If you are aware of C or C++ programming languages, then you must be aware of three standard devices STDIN, STDOUT and STDERR. Similarly, Java provides the following three standard streams −

• Standard Input − This is used to feed the data to user's program and usually a keyboard is used as standard input stream and represented as System.in.
• Standard Output − This is used to output the data produced by the user's program and usually a computer screen is used for standard output stream and represented as System.out.
• Standard Error − This is used to output the error data produced by the user's program and usually a computer screen is used for standard error stream and represented as System.err.

Following is a simple program, which creates InputStreamReader to read standard input stream until the user types a "q" −

Example

import java.io.*;

public class ReadConsole {

   public static void main(String args[]) throws IOException {

      InputStreamReader cin = null;

      try {

         cin = new InputStreamReader(System.in);

         System.out.println("Enter characters, 'q' to quit.");

         char c;

         do {

            c = (char) cin.read();

            System.out.print(c);

         } while(c != 'q');

      }finally {

         if (cin != null) {

            cin.close();

         }

      }

   }

}

Let's keep the above code in ReadConsole.java file and try to compile and execute it as shown in the following program. This program continues to read and output the same character until we press 'q' −

$javac ReadConsole.java

$java ReadConsole

Enter characters, 'q' to quit.

1

1

e

e

q

q