Wordguess program wouldn't run

[JerryRong]

This is my wordguess program but it wouldn't run. THis is similar to the hangman game on iphone

import java.util.Random;
import java.util.Scanner;
 
public class Wordguess123 {
public static String replaceCharAt(String s, int pos, char c) {
    return s.substring(0,pos) + c + s.substring(pos+1);
 }
 public static void main(String[] args){
        Scanner Jerry = new Scanner(System.in);
        Random Rong = new Random(10);
        String word;
        boolean guess = true;
        int i = 0;
        String win = "YOUWIN!!!!";
        System.out.println("This is Jerry Rong's wordguess game!");
 
        switch (Rong.nextInt()) {
 
            case 1: word = "PROPORTION";
            break;
            case 2: word = "DISTANCE";
            break;
            case 3: word = "MOMENTUM";
            break;
            case 4: word = "HELLO";
            break;
            case 5: word = "GRAPHIC";
            break;
            case 6: word = "ACCELERATION";
            break;
            case 7: word = "ENERGY";
            break;
            case 8: word = "NINJA";
            break;
            case 9: word = "FINALS";
            break;
            default: word = "CHRISTMAS";
            break;
     }
 
     int length = word.length();
     String blank = null;
     for (int h = 0; h < length; h++)
     {  blank = "_" + " ";}
 
     System.out.println(blank);
     String PeoplesGuess = blank;
 
     do {
      i++;
      String letter = in.next();
     letter = letter.toUpperCase();
     if (letter =! "!") {
 
        for (int k = 0; k < length; k++){
 
                    if (letter.charAt(0) == word.charAt(k)) {
                        current = replaceCharAt(current, k, letter.charAt(0));
                    }
 
                }
 
        if (current.equals(word)) {System.out.println("You win!!");     } 
        System.out.println ("you used" + i + "guesses ");
        System.out.println ("The secret word is" + word  );
            }
 
     else 
     {
         System.out.println("Guess the word in uppercase.");
         String guessedword = in.next();
         guess = false;
         if (guessedword.equals(word)) {System.out.println(win));}
         else System.out.println("Haha you lose");
         System.out.println ("you used" + i + "guesses ");
        System.out.println ("The secret word is" + word  );
 
 
        }
 
 
 
 
}
while ( i < 27 && guess = true);

[Mr.777]

Is there any error or exception thrown by the program? If yes, what? Elaborate your problem description.