1  import java.io.File;

  2  import java.io.FileNotFoundException;

  3  import java.io.PrintWriter;

  4  import java.util.Scanner;

  5  

  6  /**

  7     This program encrypts a file using the Caesar cipher.

  8  */

  9  public class CaesarCipher

 10  {  

 11     public static void main(String[] args) throws FileNotFoundException

 12     {  

 13        final int DEFAULT_KEY = 3;

 14        int key = DEFAULT_KEY;

 15        String inFile = null;

 16        String outFile = null;

 17  

 18        for (int i = 0; i < args.length; i++)

 19        {  

 20           String arg = args[i];

 21           if (arg.charAt(0) == '-')

 22           {  

 23              // It is a command line option

 24  

 25              char option = arg.charAt(1);

 26              if (option == 'd') { key = -key; }

 27              else { usage(); return; }

 28           }

 29           else

 30           {  

 31              // It is a file name

 32  

 33              if (inFile == null) { inFile = arg; }

 34              else if (outFile == null) { outFile = arg; }

 35              else { usage(); return; }

 36           }

 37        }

 38        if (inFile == null || outFile == null) { usage(); return; }

 39  

 40        Scanner in = new Scanner(new File(inFile));

 41        in.useDelimiter(""); // Process individual characters

 42        PrintWriter out = new PrintWriter(outFile);

 43        while (in.hasNext())

 44        {

 45           char from = in.next().charAt(0);

 46           char to = (char) (from + key);

 47           out.print(to);

 48        }

 49        in.close();

 50        out.close();               

 51     }   

 52  

 53     /**

 54        Prints a message describing proper usage.

 55     */

 56     public static void usage()

 57     {  

 58        System.out.println("Usage: java CaesarCipher [-d] infile outfile");

 59     }

 60  }