Loop through array of objects inside the class

Question

I am having some trouble looping through an array with objects, inside the class. I wrote a little demo here so you can follow:

Tank tanks[] = new Tank[2];
tanks[0] = new Tank();
tanks[1] = new Tank();
tanks[0].doStuff(tanks);

doStuff(Tank[] tanks) {
    for (int i = 0; i < tanks.length; i++) {
        if (tanks[i].equals(this)) continue;
        // Do stuff
    }
}

So, I have an array with the type Tank. Then I call the method doStuff inside the Tank class. The method takes the array and loops through it. And then I want to do stuff to every tank that is not the current instance of the class. I hope you can make sense out of my code and this description.

The problem is that I get nullPointerException for if (tanks[i].equals(this))

What am I doing wrong here?

That code won't compile anyway, as you're trying to treat a single value (Tank tanks) as an array. Please edit your question to provide a short but complete program demonstrating the problem. — Jon Skeet
– Jon Skeet, Commented May 8, 2013 at 15:45
Have you reimplemented equals method? If yes, can you show it? That's the only place I think that this null may be comming from, but only if you reimplemented it and done something wrong. The rest is clean. — Caesar Ralf
– Caesar Ralf, Commented May 8, 2013 at 15:46
@JonSkeet you can use that syntax to create an array. It's strange, but he declared it as an array in the variable name. — Caesar Ralf
– Caesar Ralf, Commented May 8, 2013 at 15:48
@RalfHoppen: The code has been edited. The original declaration had doStuff(Tank tanks). It wasn't the call site I was worried about - it was the method declaration. (It's still not a valid declaration as it has no return type, but the error I mentioned before has been fixed.) A short but complete program demonstrating the problem would still be preferable... — Jon Skeet
– Jon Skeet, Commented May 8, 2013 at 15:49

SLaks · Accepted Answer · 2013-05-08 15:44:55Z

8

That means that tanks[i] is null. (or that your overridden equals() method has a bug)

You need to check for that.

answered May 8, 2013 at 15:44

SLaks

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2 Comments

Anthony Grist Over a year ago

More likely the latter, given the relatively basic nature of the code provided.

Jon Skeet Over a year ago

@AnthonyGrist: Given that the code provided wouldn't compile (even after being silently edited), I strongly suspect it's not representative of the real code being run.

Simulant · Accepted Answer · 2013-05-08 15:47:25Z

1

if you want to compare the IDs of your object you can use == instead of .equals()

doStuff(Tank tanks) {
  for (int i = 0; i < tanks.length; i++) {
    if (tanks[i] == this) {
        continue;
      }
    // Do stuff
  }
}

answered May 8, 2013 at 15:47

Simulant

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Comments

durron597 · Accepted Answer · 2013-05-08 15:52:29Z

1

When I run this code:

public class Tank {
  public static void main(String[] args) {
    Tank tanks[] = new Tank[2];
    tanks[0] = new Tank();
    tanks[1] = new Tank();
    tanks[0].doStuff(tanks);
  }

  public void doStuff(Tank[] tanks) {
      for (int i = 0; i < tanks.length; i++) {
          if (tanks[i].equals(this)) continue;
          // Do stuff
      }
  }
}

No error happens. Therefore, you've probably overridden .equals, and that is where the NullPointerException is occurring. The other possibility is that your simple example doesn't accurately reflect where your bug is occurring.

edited May 8, 2013 at 15:52

answered May 8, 2013 at 15:46

durron597

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