Changing a variable in an Object from an ArrayList<Object>

Question

I am currently trying to create a pathfinding method for my grid based game based on the A* method algorithm. However I am having a basic problem with manipulating variables within my PathNode class instances:

public void AStarPathfinding(PathNode snakeHead, PathNode foodLocation) {

    System.out.println(food.xFood);
    System.out.println(food.yFood);

        openNodes.add(snakeHead);

        int xHead = (int) snakeSegments.get(0);
        int yHead = (int) snakeSegments.get(1);


        snakeHead.xCoordinate = (int) xHead;
        snakeHead.yCoordinate = (int) yHead;
        foodLocation.xCoordinate = (int) food.xFood;
        foodLocation.yCoordinate = (int) food.yFood;

however I am receiving null-point exception errors:

Exception in thread "Thread-2" java.lang.NullPointerException
    at ArtificialSnake.AStarPathfinding(ArtificialSnake.java:136)

which is this line:

snakeHead.xCoordinate = (int) xHead;

The idea is to set the startNode(snakeHead) to the current snake head's location.... but as suggested above I cannot work out how to modify the xCoordinate variable in the snakeHead instance of the PathNode class.

Looking at another question: Edit variables from object in ArrayList?

It suggests using setters, I have tired this however I still get null point exception errors.

Note: the Thread2 is the gameLoop thread separate from the Swing UI, the class that this pathfinding method is in is part of the same thread.

What am I missing here?

Answer 1

If you receive a null pointer error on the line snakeHead.xCoordinate = (int) xHead; this means that snakeHead is null , check that null isn't being passed into the method. If your wonderhy why a null pointer error doesn't occur at openNodes.add(snakeHead); it is because null can be added to some java collections, they don't all try to perform any operations on what you pass to them (those that do may call hashCode() , equals() or compare() depending on the collection type and it's implementation).

When you pass objects between methods and into collections/arrays you are making shallow copies (You are passing a reference to the object) so any changes you make to the object, will be visible in every other place you've passed the reference.

The only way to make a deep copy of an object (so that changes don't affect other references to the object) is to copy every member variable of the object (some classes may provide a clone method that does this).