问如何使解密方法能够将加密的字符串返回到原始字符串？

EN

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]; 

    for(int i=0; i< ALPHASIZE; i++)
        decrypt[encrypt[i]- 'A']=alpha[i];   
}
}

// 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);

}
}

Test Class
public class CaesarTryOut {


public static void main(String[] args) {


System.out.println("Please enter your message:");
Scanner scan = new Scanner(System.in);
String c = scan.nextLine();
System.out.println();

System.out.println("You entered the following message:");
System.out.println(c);
System.out.println();

Caesar cipher = new Caesar();

String code = c;



String secretEncrypt = cipher.encrypt(c);
System.out.println("Your string has been encrypted to:");
System.out.println(secretEncrypt);
System.out.println();



String secretDecrypt = cipher.decrypt(c);
System.out.println("Your message has been decrypted to:");
System.out.println(secretDecrypt);
System.out.println();

}
}