The Exception "No letters found" didn't work

After following the teacher's instruction I 've had:

(Class Prompter)

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();
//char guess = guessInput.charAt(0);
//boolean isHit =false;
try{
  isHit=game.applyGuess(guessInput.charAt(0));
  isAcceptable= true;
}
catch(IllegalArgumentException iae)
{
  System.out.printf("%s.Please try again,%n",
    iae.getMessage());
}
}
  while(!isAcceptable);

return isHit;
//boolean isHit=game.applyGuess(guess);

}

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

And in the class Game:

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 required"); } letter = Character.toLowerCase(letter); if(misses.indexOf(letter) !=-1 ||hits.indexOf(letter) !=-1) { throw new IllegalArgumentException(letter + "has already been guessed"); } return letter; }

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

public boolean applyGuess(char letter) { letter = normalizeGuess(letter); //if(misses.indexOf(letter) !=-1 ||hits.indexOf(letter) !=-1) //{ //throw new IllegalArgumentException(letter + "has already been guessed"); //} 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; } }

but the code didn't work well. I couldn't receive the Exception No letters found as I wanted. Please help me.

Thanks you guys very much.