Unexpected NullPointerException assigning arrays from constructor

Question

I can't seem to figure this out and if you guys could help me that would be awesome! I am trying to pass already made objects into a constructor so I can get all of their values.

public class Drops {
  Ship ship;
  Bullet[] bullet;
  Aliens[] aliens;
  Movement movement;

  public Drops(Ship ship,Bullet[] bull,Aliens[] alienT) {
    this.ship = ship;
    for (int a = 0; a < MainGamePanel.maxAliens;a++) {
      System.out.println(a +" " +alienT[a].x); // THIS WORKS, when nothing
                                               // is being assigned, so the values 
                                               // are being passed correctly.
      this.aliens[a] = alienT[a];
      for (int b = 0; b < MainGamePanel.maxShots;b++){
        this.bullet[b] = bull[b];
  }
    }
  }
// that is is the class, and also where the error occurs

in the main I am sending the values to the constructor like this

drop = new Drops(ship, bull, alienT);

ship is not an array bull and alienT are both arrays.

Thank you in advance!

MByD · Accepted Answer · 2012-05-09 16:17:46Z

1

You need to initialize the arrays:

Bullet[] bullet;
Aliens[] aliens;

e.g:

public Drops(Ship ship,Bullet[] bull,Aliens[] alienT){
    this.ship = ship;
    this.bullet = new Bullet[bull.length];
    this.aliens = new Aliens[alianT.length];
    // ..

Also, make sure that the loop condition takes into account the length of alienT and bull, if they are shorter than MainGamePanel.maxAliens and MainGamePanel.maxShots you'll get ArrayIndexOutOfBoundsException.

answered May 9, 2012 at 16:17

MByD

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1 Comment

Michael Over a year ago

Thank you very much, that worked. It's funny because I knew that and was messing around with that but failed miserably and put them in the damn loops! edit: And yeah maxAliens and maxShots are actually declaring the other objects array size when they get initialized in MainGamePanel.

Daniel Albert · Accepted Answer · 2012-05-09 16:19:22Z

0

You can define bull and allienT parameters as Collection<Bullet> and Collection<AllienT> respectivelly.

Then you can call this method passing an ArrayList, HashSet or your prefered collection class.

answered May 9, 2012 at 16:19

Daniel Albert

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2 Comments

Michael Over a year ago

That went a little over my head haha, I am still very new with java and programming in general, learning quickly though. A collection I am assuming is another way to just use the array called from the constructor without making new ones? Is it usable in other methods in the same class?

Daniel Albert Over a year ago

Collection is an Interface implemented by many Data Structure classes like ArrayList, HashSet, LinkedHashSet, TreeSet... You can define parameters as Collections and you can work with the implementation you decide for each case. Here is some documentation: [link] docs.oracle.com/javase/6/docs/api/java/util/Collection.html

Xavi López · Accepted Answer · 2012-05-09 16:21:17Z

0

You're getting the NPE because of the aliens and bullet member arrays are null. Make sure you're instantiating them in the constructor with the proper length:

public Drops(Ship ship,Bullet[] bull,Aliens[] alienT){
    this.ship = ship;
    this.aliens = new Aliens[alienT.length];
    this.bullet = new Bullet[bull.length];
    // ...
}

answered May 9, 2012 at 16:21

Xavi López

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