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Java Java Objects Creating the MVP Current Progress

Nuno Reis
Nuno Reis
23,956 Points

Console error

Hi, I am following this Java tutorial but I am getting this weird console error and I can't figure what's wrong with my code. Here's the error I am getting from the console:

Picked up JAVA_TOOL_OPTIONS: -Xmx128m                                                                                
Picked up _JAVA_OPTIONS: -Xmx128m                                                                                    
Picked up JAVA_TOOL_OPTIONS: -Xmx128m                                                                                
Picked up _JAVA_OPTIONS: -Xmx128m                                                                                    
Exception in thread "main" java.lang.NullPointerException                                                            
        at Game.getCurrentProgress(Game.java:24)                                                                     
        at Prompter.displayProgress(Prompter.java:20)                                                                
        at Hangman.main(Hangman.java:7)   

Hangman.java

public class Hangman {

  public static void main(String[] args) {
    // Your incredible code goes here...
    Game game = new Game("treehouse");
    Prompter prompter = new Prompter(game);
    prompter.displayProgress();
    boolean isHit = prompter.promptForGuess();
    if (isHit) {
      System.out.println("We got a hit");
    } else {
      System.out.println("Oops missed!");
    }
    prompter.displayProgress();
  }
}

Prompter.java

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);
    System.out.print("Enter a Letter: ");
    String guessInput = scanner.nextLine();
    char guess = guessInput.charAt(0);
    return game.applyGuess(guess);
  }

  public void displayProgress() {
    System.out.printf("Try to solve:  %s%n",
                      game.getCurrentProgress());
  }
}

Game.java

class Game {
  private String answer;
  private String hits;
  private String misses;

  public Game(String answer) {
    this.answer = answer;
  }

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

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

}

Can anyone provide some help here? Thanks :)

1 Answer

On this line:

if (hits.indexOf(letter) != -1 ) {

You're calling the method indexOf() on the String 'hits'. But the variable 'hits' is only assigned to a string once you run applyGuess() and a letter matches. Before that the variable 'hits' does not have any string assigned to it. If you try to rund indexOf() on it, it will fail, because there is no string object to run this method. So you get a NullPointerException (because the variable points to a Null).

Two possible solutions:

  1. Initalize the variable 'hits' in the constructor by assigning it to an empty string
  2. Change your if condition to first check that the variable 'hits' is not null. If it's not null, then check if the string contains the letter.
Nuno Reis
Nuno Reis
23,956 Points

Ah right, gotcha! Thanks Livia :)