NullPointerException when trying to add an object to a PriorityQueue

Question

i keep getting a null pointer exception when trying to add an object to a priority queue

i initialize the queue:

private PriorityQueue<NodeObject> nodes;

and here i try to add to it:

NodeObject childNode = new NodeObject(child, 1);
nodes.add(childNode);

why doesn't this work? i know my NodeObject is not null because i create it right before i add it.

Answer 1

You haven't initialized the queue.

nodes = new SomePriorityQueue();

Answer 2

The problem is that you forgot to initialize your priority queue nodes. Change it to:

private PriorityQueue<NodeObject> nodes = new PriorityQueue<NodeObject>();

Or explicitly allocate nodes (as in nodes = new PriorityQueue<NodeObject>();) in each of your constructors. Keep in mind that in Java, any type that isn't a primitive is actually an implicit pointer, so while the pointer nodes has been initialized to null, it has not been initialized to point to something.

Answer 3

You didn't actually create a priority queue; nodes == null. This causes NullPointerException to be thrown when you try to invoke nodes.add.

The way to fix this is by actually constructing a PriorityQueue<NodeObject>, and assigning the reference to it to nodes. You can do this in the declaration as follows:

private PriorityQueue<NodeObject> nodes = new PriorityQueue<NodeObject>();
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

JLS 4.12.5 Initial Values of Variables

• Each class variable, instance variable, or array component is initialized with a default value when it is created:
  • For all reference types, the default value is null.