空指针异常和地图

Question

很抱歉，如果这是一个愚蠢的问题，但我对java很新。基本上我们正在学习java类中的接口和映射。我创建了一个非常简单的接口，并且还创建了实现该接口的其他类。在我作为主要演员的班级中，我正在创建一个地图，我已经创建了这些实例，如下所示。我得到了NPE而且我不确定为什么。

private Scanner input;
private Map<String, Command> 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<String, Command>();
    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();
    }
}

}

错误出现在map map.get（word）.execute（）;

上

我确定这是显而易见的事情，但我对编码很陌生，所以任何帮助都会受到赞赏！