title: Null Pointer Exception and maps
body: <p>Sorry if this is a stupid question, but I'm very new to java.  Basically we are learning about interfaces and maps right now in my java class.  I created an interface that is really simple, and have also created other classes that implement that interface.  In my class that is a main actor, I am creating a map and I've created instances of these as you can see below.  I'm getting a NPE and I am not sure why.  </p>
  
  <pre><code>private Scanner input;
  private Map&lt;String, Command&gt; map;
  
  //~ Constructors ..........................................................
  // ----------------------------------------------------------
  /**
   * Creates a new Rocket object that reads commands
   * from a predefined URL (the URL is in the lab assignment).
   */
  public Rocket()
  {
      this(IOHelper.createScannerForURL(
              "http://courses.cs.vt.edu/~cs1114/Fall2012/rocket-commands.txt")
              );
  
  }
  
  // ----------------------------------------------------------
  /**
   * Creates a new Rocket object that reads commands
   * from the given scanner.
   * @param scanner The scanner to read commands from.
   */
  public Rocket(Scanner scanner)
  {
      input = scanner;
      map = new HashMap&lt;String, Command&gt;();
      ForwardCommand forward = new ForwardCommand(this);
      map.put("forward", forward);
      LeftCommand left = new LeftCommand(this);
      map.put("left", left);
      RightCommand right = new RightCommand(this);
      map.put("right", right);
  }
  
  //~ Methods ...............................................................
  // ----------------------------------------------------------
  /**
   * Reads one word from the scanner (if there is any), and executes
   * the corresponding command.  If the scanner has no words remaining,
   * then nothing happens.
   */
  public void act()
  {
      if (input.hasNext())
      {
          String word = input.next();
  
          map.get(word).execute();
      }
  }
  </code></pre>
  
  <p>}</p>
  
  <p>The error is coming on the line map.get(word).execute();</p>
  
  <p>I'm sure it is something obvious, but I am very new to coding so any help would be appreciated!</p>