Console says I can't convert int to string while I'm not using int

I’m getting error at

String temporarytext = (b1 + b2 + b3 + b4 + b5 + b6 + b7 + b8 + b9 + b10 + b11 + b12);

It says I can’t convert int to string while I’m not using int but I’m not trying to?
I’m only trying to conver char to string

import java.util.Scanner;

public class Encrypt
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        
        System.out.println("Plaintext");
        
        String word1 = scan.next();
        String word2 = scan.next();
        String word3 = scan.next();
        
        String plaintext = word1 + word2 + word3;
        
        char a1 = plaintext.charAt(0);
        char a2 = plaintext.charAt(1);  
        char a3 = plaintext.charAt(2);  
        char a4 = plaintext.charAt(3);  
        char a5 = plaintext.charAt(4);
        char a6 = plaintext.charAt(5);  
        char a7 = plaintext.charAt(6);  
        char a8 = plaintext.charAt(7);  
        char a9 = plaintext.charAt(8);  
        char a10 = plaintext.charAt(9); 
        char a11 = plaintext.charAt(10);    
        char a12 = plaintext.charAt(11);
        
        char b1, b2, b3, b4, b5, b6, b7, b8, b9, b10, b11, b12;
        
        if (a1 == (char) 120)
            b1 = (char) 97; 
        if (a1 == (char) 121)
            b1 = (char) 98; 
        if (a1 == (char) 122)
            b1 = (char) 99; 
        else
            b1 = ((char)(3 + (int) a1));
        
        if (a2 == (char) 120)
            b2 = (char) 97; 
        if (a2 == (char) 121)
            b2 = (char) 98; 
        if (a2 == (char) 122)
            b2 = (char) 99; 
        else
            b2 = ((char)(3 + (int) a2));
        
        if (a3 == (char) 120)
            b3 = (char) 97;
        if (a3 == (char) 121)
            b3 = (char) 98; 
        if (a3 == (char) 122)
            b3 = (char) 99; 
        else
            b3 = ((char)(3 + (int) a3));    
        
        if (a4 == (char) 120)
            b4 = (char) 97; 
        if (a4 == (char) 121)
            b4 = (char) 98; 
        if (a4 == (char) 122)
            b4 = (char) 99;     
        else
            b4 = ((char)(3 + (int) a4));    
        
        if (a5 == (char) 120)
            b5 = (char) 97;
        if (a5 == (char) 121)
            b5 = (char) 98; 
        if (a5 == (char) 122)
            b5 = (char) 99;     
        else
            b5 = ((char)(3 + (int) a5));    
        
        if (a6 == (char) 120)
            b6 = (char) 97;
        if (a6 == (char) 121)
            b6 = (char) 98; 
        if (a6 == (char) 122)
            b6 = (char) 99;     
        else
            b6 = ((char)(3 + (int) a6));    
        
        if (a7 == (char) 120)
            b7 = (char) 97;
        if (a7 == (char) 121)
            b7 = (char) 98; 
        if (a7 == (char) 122)
            b7 = (char) 99;     
        else
            b7 = ((char)(3 + (int) a7));    
        
        if (a8 == (char) 120)
            b8 = (char) 97; 
        if (a8 == (char) 121)
            b8 = (char) 98;
        if (a8 == (char) 122)
            b8 = (char) 99; 
        else
            b8 = ((char)(3 + (int) a8));    
        
        if (a9 == (char) 120)
            b9 = (char) 97; 
        if (a9 == (char) 121)
            b9 = (char) 98; 
        if (a9 == (char) 122)
            b9 = (char) 99; 
        else
            b9 = ((char)(3 + (int) a9));    
        
        if (a10 == (char) 120)
            b10 = (char) 97;
        if (a10 == (char) 121)
            b10 = (char) 98;
        if (a10 == (char) 122)
            b10 = (char) 99;        
        else
            b10 = ((char)(3 + (int) a10));  
        
        if (a11 == (char) 120)
            b11 = (char) 97;
        if (a11 == (char) 121)
            b11 = (char) 98;
        if (a11 == (char) 122)
            b11 = (char) 99;        
        else
            b11 = ((char)(3 + (int) a11));  
        
        if (a12 == (char) 120)
            b12 = (char) 97;
        if (a12 == (char) 121)
            b12 = (char) 98;    
        if (a12 == (char) 122)
            b12 = (char) 99;    
        else
            b12 = ((char)(3 + (int) a12));                      
        
        String temporarytext = (b1 + b2 + b3 + b4 + b5 + b6 + b7 + b8 + b9 + b10 + b11 + b12);
        
        String cipherword1 = temporarytext.substring(0, word1.length());
        String cipherword2 = temporarytext.substring(word1.length() + 1, word1.length() + word2.length() + 1);
        String cipherword3 = temporarytext.substring(word1.length() + word2.length() +2 , 14);
        
        String ciphertext = cipherword1 + " " + cipherword2 + " " + cipherword3;
        
        System.out.println("Ciphertext");
        System.out.println(ciphertext);     
    }
}   

>Solution :

Your code needs a lot of work, but to answer the question narrowly, you need to sort of "kick off" the string concatenation by starting with a string. Below I just use an empty string "" to get started. I also believe you need to convert your ints to chars to let the compiler know what you want to do.

   public static void main( String[] args ) {
      int b1 = 97;
      int b2 = 98;
      int b3 = 99;
      
      String s = "" + (char)b1 + (char)b2 + (char)b3;
      System.out.println( s );
   }