Im getting stuck in an endless loop and cant figure out where i went wrong.

class Game {
  public static final int MAX_MISSES = 7;
  private String answer;
  private String hits;
  private String misses;

  public Game(String answer){
    this.answer = answer.toLowerCase();
    hits = "";
    misses = "";
  }

  private char normalizeGuess(char letter){
   if (! Character.isLetter(letter)){
     throw new IllegalArgumentException("A letter is requierd");
   }
    letter = Character.toLowerCase(letter);
     if (misses.indexOf(letter) != -1 || hits.indexOf(letter) != -1){
     throw new IllegalArgumentException(letter + " has allready been guessed"); 
    }
    return letter; 
  }

 public boolean applyGuess (String letters){
    if (letters.length() == 0){
      throw new IllegalArgumentException("No letter found");
    }
    return  applyGuess(letters.charAt(0));
  }  


  public boolean applyGuess (char letter){
    letter = normalizeGuess(letter);
    boolean isHit = answer.indexOf(letter) != -1;
    if (isHit){
      hits += letter;
    } else {
      misses += letter;
    }
    return isHit;
  }



  public int getRemainingTries(){
    return MAX_MISSES - misses.length();
  }

  public String getCurrentProgress(){
    String progress = "";
    for (char letter : answer.toCharArray()){
      char display = '-';
      if ((hits.indexOf(letter) != -1)) {
        display = letter;
      }
          progress += display;
          }

          return progress;

  }

}

import java.util.Scanner;
class Prompter {
 private Game game;


  public Prompter(Game game){
    this.game = game;
  }

  public boolean promptForGuess(){
    Scanner scanner = new Scanner(System.in);
    boolean isHit = false;
    boolean isAcceptable = false;

    do {
    System.out.print("Enter a letter:  ");
    String guessInput = scanner.nextLine();


    try {
      isHit = game.applyGuess(guessInput);
    } catch (IllegalArgumentException iae) {
      System.out.println(iae.getMessage());
    }

    }while (! isAcceptable);

    return isHit;
   }

  public void displayProgress() {
    System.out.printf("You have %d tries left to solve:  %s\n", game.getRemainingTries(), game.getCurrentProgress());
  }

}

public class Hangman {

  public static void main(String[] args) {
    // Your incredible code goes here...
    Game game = new Game("treehouse");
   Prompter prompter = new Prompter(game);
    while (game.getRemainingTries() > 0) {
    prompter.displayProgress();
    prompter.promptForGuess();
    }  
  }
}