[tennernote17]

I have basically been instructed to write a program which should encrypt a given string by input from the user and generate a random encryption character for each letter that has been input while ensuring that the same character appearing more than once in the original is replaced by the same character in the coded string. I must also ensure that no two characters in the original are coded as the same character in the coded string, in order to enable successful decryption. However the decryption gives me back a completely different string to what I had in the first place, here is my code:

public class Caesar
{
    public static final int ALPHASIZE = 26;             // upper case alphabets
    public static final char[] alpha = {'A','B','C','D',
        'E','F','G','H','I','J','K','L','M','N','O','P',
        'Q','R','S','T','U','V','W','X','Y','Z'};
 
 
    protected char[] encrypt = new char [ALPHASIZE];    // encryption array
    protected char[] decrypt = new char [ALPHASIZE];    // decryption array
 
    // constructor that initializes the encryption and decryption arrays
    
    Random randomGen = new Random();
    
    public Caesar() {
    
    int random = randomGen.nextInt(ALPHASIZE);
    
    {
        for(int i=0; i< ALPHASIZE; i++)
            encrypt[i]=alpha[(i+random)%ALPHASIZE];  // rotate alphabet by 3 places
        
        for(int i=0; i< ALPHASIZE; i++)
            decrypt[encrypt[i]- 'A']=alpha[i];   // decrypt is reverse of encrypt
    }
    }
   
    // encryption method
    public String encrypt(String secret)
    {
        char[] mess = secret.toCharArray();     // the message array
        for(int i=0; i<mess.length; i++)        // encryption loop
            if(Character.isUpperCase(mess[i]))  // a letter to change
                mess[i]=encrypt [mess[i]-'A'];  // use letter as index
            else  if(Character.isLowerCase(mess[i]))  // a letter to change
                mess[i]=(new String(""+encrypt [mess[i]-'a'])).toLowerCase().charAt(0);  // use letter as index
        return new String (mess);
    }
 
    // decryption method
    public String decrypt (String secret)
    {
        char[] mess = secret.toCharArray();
        for(int i=0; i<mess.length; i++)
            if (Character.isUpperCase(mess[i]))
                mess[i]=decrypt[mess[i]-'A'];
             else  if(Character.isLowerCase(mess[i]))  // a letter to change
                mess[i]=(new String(""+decrypt [mess[i]-'a'])).toLowerCase().charAt(0);  // use letter as index
        return new String (mess);
 
    }
}

Any help will be greatly appreciated.

[Scleros]

Passing some strings to your code seems to work. What am I missing?

/*
** Compiled with Oracle JDK 1.7.0_45
*/

import java.util.Random;

public class MyCrypto {

	public static void main(String args[]) {

		String[] MySecrets = {"www.AskMeHelpDesk.com", "AskMeHelpDesk is AWESOME", "Get Help at AskMeHelpDesk"};
		String MyEncryptedSecret = "";
		String MyDecryptedSecret = "";

		Caesar MyCrypto = new Caesar();

		System.out.println("\nMyCrypto\n--------");
	
		for(int i=0; i< MySecrets.length; i++) { 

			System.out.println("\n      MySecret[" + (i+1)+ "]: " + MySecrets[i]);

			// Encrypting...
			MyEncryptedSecret = MyCrypto.encrypt(MySecrets[i]);
			System.out.println("MyEncryptedSecret: " + MyEncryptedSecret);

			// Decrypting...
			MyDecryptedSecret = MyCrypto.decrypt(MyEncryptedSecret);
			System.out.println("MyDecryptedSecret: " + MyDecryptedSecret);
		}

		return;
	}


	public static class Caesar {

	    public static final int ALPHASIZE = 26;             // upper case alphabets
	    public static final char[] alpha = {'A','B','C','D',
	        'E','F','G','H','I','J','K','L','M','N','O','P',
	        'Q','R','S','T','U','V','W','X','Y','Z'};
 
 
	    protected char[] encrypt = new char [ALPHASIZE];    // encryption array
	    protected char[] decrypt = new char [ALPHASIZE];    // decryption array
	 
	    // constructor that initializes the encryption and decryption arrays
	    
	    Random randomGen = new Random();
	    
	    public Caesar() {
	    
	    	int random = randomGen.nextInt(ALPHASIZE);
	    
	        for(int i=0; i< ALPHASIZE; i++)
	            encrypt[i]=alpha[(i+random)%ALPHASIZE];  // rotate alphabet by 3 places
	        
	        for(int i=0; i< ALPHASIZE; i++)
	            decrypt[encrypt[i]- 'A']=alpha[i];   // decrypt is reverse of encrypt
	    }
   
	    // encryption method
	    public String encrypt(String secret)
	    {
	        char[] mess = secret.toCharArray();     // the message array
	        for(int i=0; i<mess.length; i++)        // encryption loop
	            if(Character.isUpperCase(mess[i]))  // a letter to change
	                mess[i]=encrypt [mess[i]-'A'];  // use letter as index
	            else  if(Character.isLowerCase(mess[i]))  // a letter to change
	                mess[i]=(new String(""+encrypt [mess[i]-'a'])).toLowerCase().charAt(0);  // use letter as index
	        return new String (mess);
	    }
 
	    // decryption method
	    public String decrypt (String secret)
	    {
	        char[] mess = secret.toCharArray();
	        for(int i=0; i<mess.length; i++)
	            if (Character.isUpperCase(mess[i]))
	                mess[i]=decrypt[mess[i]-'A'];
	             else  if(Character.isLowerCase(mess[i]))  // a letter to change
	                mess[i]=(new String(""+decrypt [mess[i]-'a'])).toLowerCase().charAt(0);  // use letter as index
	        return new String (mess);
 
	    }
	}
}