About Console Input

Reading from System.in

What is System.in ?

It is a system variable. Its data type is InputStream. From the documentation of java.io.InputStream, the simplest method to read a byte is the read() method.

To read a line from the console, a while loop can be used to repeatedly read character until the linefeed character is read.

// read a line from console input character by character
public static void main(String[] args) throws Exception
{
  String s="";
  byte[] buffer=new byte[1];
  while (true)
  {
    int n=System.in.read();
    char ch=0;      
    if (n>0) {
      ch=(char) n; 
      s += ch;
    }
    if (ch=='\n') break;
  }
  System.out.println("Input String="+s);
}

Reading System.in character by character is not efficient. Java does provide wrapper for InputStream so that reading a line is more simple. The following program wraps System.in with a InputStreamReader, then wraps the InputStreamReader with a BufferedReader object. Since BufferedReader supports the readLine() method, it is a more high level and thus simpler implementation.

// read a line from Console using BufferedReader
public static void main(String[] args) throws Exception
{
  java.io.BufferedReader br=new java.io.BufferedReader(
    new java.io.InputStreamReader(System.in));
  String line=br.readLine();
  System.out.println(line);
}

Command Line Interpreter

Let's complete the demonstration by writing a simple command interpreter. The interpreter supports only three commands :

1. time
   
2. date
   
3. exit
   

The commands will display the current time, current date and exit the program respectively. The program won't stop until you type 'exit'. Anything other than the above commands will result in an error message.

/******************************************************************************
* File : ConsoleInput.java
* Author : http://java.macteki.com/
* Description :
*   Console Input Demonstration (A simple command line interpreter)
* Tested with : JDK 1.6
******************************************************************************/

class ConsoleInput
{

  // a simple "command interpreter"
  public static void doCommand(String line)
  {
    if (line.equals("time"))
    {
      // display current time
      java.util.Date date=new java.util.Date();
      java.text.SimpleDateFormat f=new java.text.SimpleDateFormat("HH:mm");
      String time=f.format(date);
      System.out.println(time);
    }
    else if (line.equals("date"))
    {
      // display current time
      java.util.Date date=new java.util.Date();
      java.text.SimpleDateFormat f=new java.text.SimpleDateFormat("yyyy-MM-dd");
      String dateString=f.format(date);
      System.out.println(dateString);
    }
    else if (line.equals("exit"))
    {
      System.exit(1);  // quit the program
    }
    else
    {
      System.out.println("Unknown command !");
    }
  }

  public static void main(String[] args) throws Exception
  {
    while (true)
    {
      java.io.BufferedReader br=new java.io.BufferedReader(
        new java.io.InputStreamReader(System.in));
      String line=br.readLine();
      doCommand(line);
    }
  }
}

Thanks for reading. Comments are welcome.