PriorityQueue poll() throwing NullPointerException

Question

I am working with a priority queue in Java for the first time and I can't for the life of me understand what I am doing that is leading to the exception. I'm attempting to implement an ant colony type solution to the traveling salesman problem. The following is the only code being called for my AntColony class.

public AntColony(TSPInstance p) {
    PriorityQueue<Ant> ants = new PriorityQueue<Ant>(new AntComparator());
    size = p.getDimension();
    for (int i = 0; i < size; i++) {
        ants.offer(new Ant(p));
    }
    shortestTour = Integer.MAX_VALUE;
}

public void nextMove() {
    ants.poll();
}

The code that I'm running afterwards just as a test is as follows (just in a main method).

AntColony a = new AntColony(p);
a.nextMove();

The a.nextMove() throws a NullPointerException at the ants.poll() part, but yet if I change the constructor to (for debugging purposes)

public AntColony(TSPInstance p) {
    PriorityQueue<Ant> ants = new PriorityQueue<Ant>(new AntComparator());
    size = p.getDimension();
    for (int i = 0; i < size; i++) {
        ants.offer(new Ant(p));
    }
    ants.poll(); //ADDED THIS
    shortestTour = Integer.MAX_VALUE;
}

and then just do

AntColony a = new AntColony(p);

I don't get an exception. I'm struggling to understand how I'm getting an exception from ants.poll(), but yet when I call it from the constructor everything works. Any help with this would be appreciated. There's a lot of code for various things in this project, so I didn't think uploading it all would help anybody so let me know if there's something I should include, but I don't see how the problem could lie outside these two bits of code.

Added: Actual exception

Exception in thread "main" java.lang.NullPointerException
at data_structures.AntColony.nextMove(AntColony.java:25) (the ants.poll() part)
at algorithms.ACTest.main(ACTest.java:6) The a.nextMove() part

It seems you don't have `ants` declared in your `nextMove` method or at class level. — alayor, Apr 20 '17 at 14:31
Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Deepesh Choudhary, Apr 20 '17 at 16:48

score 3 · Accepted Answer · answered Apr 20 '17 at 14:32

The ants variable in your AntColony constructor is a local variable. So when you exit the constructor, it no longer exists. Apparently the ants variable that your nextMove method is calling, is a class member.

You need to change your constructor to have:

    // initialize the class member, not a local instance.
    ants = new PriorityQueue<Ant>(new AntComparator());

alayor · Answer 2 · 2017-04-20T14:41:26.120

0

You can just remove the PriorityQueue declaration in your AntColony constructor.

public AntColony(TSPInstance p) {
    ants = new PriorityQueue<Ant>(new AntComparator());
    size = p.getDimension();
    ...
}

UPDATE: The cause for your NullPointerException is that you are not initializing your ants property in your constructor but you are creating a new local ants instead. So the ants object in nextMove method has the same value as you provided in your class level declaration, which it's probably null.

edited Apr 20 '17 at 14:41

answered Apr 20 '17 at 14:34

alayor

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You're right but should explain what is the issue and why your fix works. Presented like you said, it seems a bit magical (you've got one too many word instead of "you declared ants as a local variable") – Jeremy Grand Apr 20 '17 at 14:36

PriorityQueue poll() throwing NullPointerException

2 Answers2