CCSU  Spring, 1999

Project 4

Posting Date:  February 2, 1999
Due Date:      April 23, 1999

Relevant Demos:  Demo 15 (ArrayDemo.java) and Demo 17 (MartianToEnglishApplet.java)

This project comes in only 1 part.  It covers the material covered in Chapter 6 
on arrays.

Background:

   Caesarean ciphers are a class of codes related to ones invented by the 
ancient Romans for transmitting battlefield messages.  The original coding 
method involved wrapping a leather strip around a round stick and writing the 
message down the leather wrapped stick.  But in modern terminology, a Caesarean 
code is a code which depends on shifting letters of the alphabet a fixed 
distance from their original position.

   The coding method can best be illustrated by an example.  Suppose we have

Key: 3
Plain Text Message: Attack the Gauls at dawn!

First, ALL letters are changed to uppercase, and all blanks and punctuation 
marks are removed from the plain text message, giving us:

Key: 3
Plain Text: ATTACK THE GAULS AT DAWN

Next, two copies of the alphabet are constructed, with the second one shifted 
by the key length, which in this case is 3:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
   ABCDEFGHIJKLMNOPQRSTUVWXYZ

Then the overhang on the lower copy is wrapped around, giving us:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
XYZABCDEFGHIJKLMNOPQRSTUVW

Finally, the lower copy of the alphabet is used to put the message into code, 
giving us:

Key: 3
Coded Text: XQQXZHQEBDXRIPXQAXTK

To decode, just reverse the process:

Key: 3
Plain Text: ATTACKTHEGAULSATDAWN


Description:

   Write an Applet to put a message input by the user into code, using a 
Caesarian cipher.  Your Applet Panel should contain a TextField where the
user types in the message, and a second text field where the user types in
the key.  Your Panel should also contain two buttons, titled "Code" and 
"Decode".  When the user clicks the Code Button, the encoded message should 
appear in a third TextField (rather than being drawn directly on the Applet 
Panel).  On the other hand, when the user clicks the Decode button, the decoded 
message should appear in the TextField.

   Note that there are no responses to pressing the Enter key in this Applet. 
All actions are carried out as a result of pushing one of the two Buttons on 
the Applet Panel.

   Note also that the sender should be able type in the key as well as the 
message.  The sender should be expected to have a properly prepared message, 
all in upper case with no blanks and no numbers.  The key should be an integer 
between the values of -25 and 25.

   Note that the receiver could decode the message simply by using your program 
and the negative of the original key.  Test to be sure this works.  (If it does 
not, you made a mistake.)
  
Hints:

   To convert a letter to a number from 0 to 25, try using  num = (int) letter 
- (int) 'A'.  To wrap the alphabet around, try using the mod operation, as in  
codeNum = (num + key) % 26.  Because the % operation never works properly on 
negative numbers in any computer language, you first have to test to be sure 
num + key > 0.  So the code looks something like

   codenum = num + key;
   if( codenum < 0 ) codenum = codenum + 26;
   codenum = codenum % 26;

To convert a code number to a code letter, try using  codeLetter = (char) 
(codeNum + (int) 'A');.


Notes:

(1) Your files MUST be named CodeApplet.java, and CodeApplet.html so I 
can easily find them.

(2) Each of your programs must begin with a remark box as described in 
project 1.

(3) Each method in your program must begin with a remark box as illustrated in 
the demos.

Due:  Friday, April 23, 1999 AT THE BEGINNING OF CLASS.