Standard Input, Output, and Error Streams

JVM Tutorials - Herong's Tutorial Examples

∟java.lang.System Class - The Operating System

∟Standard Input, Output, and Error Streams

This section provides a tutorial example on how to use Standard Input, Output, and Error Streams provided by the java.lang.System class.

The first thing to learn about the java.lang.System class is how to use standard input, output and error streams provided as System.in, System.out, and System.err properties.

Here is tutorial example program testing standard input, output and error streams:

/* SystemInOut.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class SystemInOut {
   public static void main(String[] a) {

      // Standard input and output streams
      java.io.PrintStream out = System.out;
      java.io.InputStream sin = System.in;
      java.io.PrintStream err = System.err;

      // Converting the standard input stream into a reader
      java.io.BufferedReader in =
        new java.io.BufferedReader(new java.io.InputStreamReader(sin));

      while (true) {
         // Writing to the standard output stream
         out.println("Please enter a number:");

         try {
            // Reading from the standard input reader
            String line = in.readLine();
            double input = Double.parseDouble(line);
            double output = Math.sqrt(input);
            out.println("SQRT("+input+") = "+output);

         } catch (Exception e) {
            out.println("Caught an exception. See the error message!");

            // Writing to the standard error stream
            err.println("Exception detail:");
            err.println(e.toString());
         }
      }
   }
}

Here is a test session of this sample program with HotSpot 13 on my macOS computer:

herong> java SystemInOut

Please enter a number:
25
SQRT(25.0) = 5.0
Please enter a number:
4
SQRT(4.0) = 2.0
Please enter a number:
Four
Caught an exception. See the error message!
Exception detail:
java.lang.NumberFormatException: For input string: "Four"
Please enter a number:
(press Ctrl-C)

The test result tells me that:

On Windows systems, standard input, output, and error streams are all connected to the JVM console by default.
Pressing Ctrl-C terminates the application immediately.

If you want to separate the standard error stream to a log file, you can use the Windows redirection symbol 2> as shown in the example below:

herong> java SystemInOut 2> error.log

Please enter a number:
25
SQRT(25.0) = 5.0
Please enter a number:
4
SQRT(4.0) = 2.0
Please enter a number:
Four
Caught an exception. See the error message!
Please enter a number:
(press Ctrl-C)

herong> more error.log
Exception detail:
java.lang.NumberFormatException: For input string: "Four"

Takeaways:

Standard error stream can be redirected to a file using 2> at the command line.
Standard input stream can be redirected from a file using < at the command line. You can test this easily.
Standard output stream can be redirected to a file using > at the command line. You can test this easily.

Note that when I tested the same program with HotSpot 1.6 on my Windows XP system, the program responded differently on Ctrl-C input:

herong> java SystemInOut

Please enter a number:
25
SQRT(25.0) = 5.0
Please enter a number:
4
SQRT(4.0) = 2.0
Please enter a number:
Four
Caught an exception. See the error message!
Exception detail:
java.lang.NumberFormatException: For input string: "Four"
Please enter a number:
(press Ctrl-C)
Caught an exception. See the error message!
Exception detail:
java.lang.NullPointerException
Please enter a number:

The test result tells me that Ctrl-C did not terminate the application immediately. It caused the readLine() returning a null, which throws an exception on the Double.parseDouble(line) call.