Null pointer exception error that I am having trouble figuring out?

Question

I can't figure out how to fix my null pointer. My holder (Holder Pattern class) is undefined - I'm not sure how to fix it. I am still pretty new to programming. Thanks for any ideas on how to go about fixing this!

public Goose (JPanel container, GooseBehavior behavior, BehaviorHolder holder) {
    super(container); 
    _holder = holder;
    double rnd = Math.random(); //local variable to create random angle
    //System.out.println(rnd);
    this.setRotation(rnd);//sets the rotation angle to a random angle
    this.setSize(25, 20); //sets size 
    this.setFillColor(java.awt.Color.RED); //sets color 
    this.setWrapping(true); //sets wrapping to true
    _gooseBehavior = behavior; //stores _gooseehavior
    _gooseBehavior = _holder.getBehavior();
}

public void react() {
    _gooseBehavior.stop(); //tells the current _gooseBehavior to stop
    _holder.getBehavior(); //gets the stored behavior from the holder
    _gooseBehavior = _holder.getBehavior(); //sets the new value to one stored in holder
    _gooseBehavior.setTarget(this); //sets the target on goose
    _gooseBehavior.start(); //starts the behavior

}

Stacktrace:

Exception in thread "main" java.lang.NullPointerException
at Animal.Goose.<init>(Bee.java:26)
at Animal.DrawingPanel.<init>(DrawingPanel.java:30)
at Animal.ControlPanel.<init>(ControlPanel.java:27)
at Animal.App.<init>(App.java:28)
at Animal.App.main(App.java:39)

Here's the code where I instantiate BehaviorHolder in my Control Panel (top level object), and then I just have it stored in my drawingPanel, Goose class, and BehaviorButtons so that its associated with the three.

public ControlPanel() {
    super();
    this.setLayout(new BorderLayout());//sets a new BorderLayout
    _drawingPanel = new DrawingPanel(_holder); 
    _moveRandomly = new MoveBehavior();
    _doNothing = new StopBehavior();
    _kingGoose = new FollowBehavior(_kingGoose);
    _holder = new BehaviorHolder(_moveBehavior);

Answer 1

Its hard to say without a stack trace and incomplete code example, but there's multiple potential problems with your code:

public Goose (JPanel container, GooseBehavior behavior, BehaviorHolder holder) {
    super(container); 

    // removed some code for clarity

    _gooseBehavior = behavior;                     // <<- You store behaviour here
    _gooseBehavior = _holder.getBehavior();        // <<- then immediately override it
}

public void react() {
    _gooseBehavior.stop();
    _holder.getBehavior();                  // <-- this isn't getting assigned to anything
    _gooseBehavior = _holder.getBehavior(); // A duplicate assignment, previously done in constructor. Are you changing holder's behaviour outside of the display class?

    _gooseBehavior.setTarget(this); //sets the target on goose
    _gooseBehavior.start(); //starts the behavior    
}

I would guess _gooseBehaviour is null, but you can help us answer your question if you include the full stack trace (The error that was printed out that let you know you had a null pointer exception).

EDIT

Based off your NPE I'll bet dollar's to donuts that your holder is getting passed into the constructor as a null. Inspect it's value in your debugger or print out its value to verify.

Answer 2

It can help to explicitly validate the constructor/method parameters, and throw a more informative exception if there's something wrong:

if (behavior == null)
    throw new NullPointerException("behavior is null");
if (holder == null)
    throw new NullPointerException("holder is null");

That way:

1. If the problem is a null parameter you'll immediately not only know that it's a parameter problem, but which parameter is the problem.
2. If you get a NullPointerException without any added info you know immediately that the problem isn't a null parameter.

Then you can add further validation of the parameters once you know they aren't null :

if (holder.getBehavior() == null)
    throw new IllegalArgumentException("holder's behavior is null");