I don't get what's wrong??

public class Game {
  public static final int MAX_MISSES = 7;
  private String mAnswer;
  private String mHits;
  private String mMisses;
  private String letter;

  public Game(String answer) {
    mAnswer = answer;
    String mHits = "";
    String mMisses = "";

  }

  private char validate(char letter){
   if (! Character.isLetter(letter)){
    throw new IllegalArgumentException("a letter is required"); 
   }
    letter = Character.toLowerCase(letter);
    if (mMisses.indexOf(letter) >=0 || mHits.indexOf(letter) >=0) {
     throw new IllegalArgumentException(letter + " has already been guessed");  
    }
    return letter;
  }

  public boolean applyGuess(char letter) {
    letter = validateGuess(letter);
    boolean isHit = mAnswer.indexOf(letter) >= 0;
    if (isHit) {
      mHits += letter;
    } else {
      mMisses += letter;
    }
    return isHit;
  }

  public String getCurrentProgress() {
    String progress = "";
    for (char letter : mAnswer.toCharArray()) {
      char display = '-';
      if (mHits.indexOf(letter) >= 0) {
        display = letter;
      }
      progress += display;
    }
    return progress;
  }

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