Encoder output error heed help

hey im working on this program that shifts the Unicode value by the users choice but somewhere in my code there is a error and i can't find it when you attempt to shift it adds 2 letters and when i attempted to fix that it shifted wrong on the 5 spaces this is my last assignment for the semester and i would really like to get it out of the way to help program for a robot convention heres the code

Code :

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import java.util.Scanner;
public class EncoderDecpder {
 
 
	private static void messageEncoder()
{
	System.out.println ("Enter the message you want to encode");
	Scanner sc = new Scanner(System.in);
	String message = sc.nextLine();
	System.out.println("what number would you like to decode number to be: ");
    int shift = sc.nextInt();
     // encodes message
     for (int loop = 0;loop < message.length();loop++)
     {
         //int encoderCh= (int)message.charAt (loop);
         char tempString = message.charAt(loop);
         char replaceChar = (char)((int)(tempString + shift));
         message.replace(tempString, replaceChar);
 
        	 System.out.print(replaceChar);
        	// System.out.println();
        	// System.out.print(tempString);
 
     }
     System.out.println("to decode run the number: "+shift+" through the decoder");   
}
 
 
	private static void messageDecoder()
	{
		/*
		 * Need to finish encoder before starting decoder
		 * */
 
	}
 
	public static void main(String[] args) {
		 boolean end = false;
		    do {
		Scanner command = new Scanner(System.in);
		System.out.println ("COMMAND MENU\n1:Message Encoder\n2:Message Decoder\n3:Termanate Program");
		int menuChoice = command.nextInt();
		if (menuChoice == 1)
	    {messageEncoder();}
	    if (menuChoice == 2)
	    {messageDecoder();}
	    if (menuChoice == 3)
	    {end = true;}
		    }while(end == false);
 
	}
 
}

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