Why is a nullpointerexception occuring?

Question

I am implementing a priority queue using a stack, but a NullPointerException is occurring and I can't figure out why. I am trying to add an integer "1" by using the push method in the MyStack class. The error occurs at the add statement.

class MyStack<E> {
    private PriorityQueue<MyNode<E>> pq;
    int p =0;

    public void push(int j){
        p++;
        pq.add(new MyNode(j,p)); // error detected here
    }

    public MyNode pop(){
        if(isEmpty()){
            return null;
        }
        return pq.poll();
    }

    public boolean isEmpty() { 
        return pq.isEmpty();
    }
}

Edited: Reflects problem encountered in a proper manner :)

Answer 1

If the code is complete it is simple: The member pq of your class MyStack is never initialized.

Answer 2

Change your line private PriorityQueue<MyNode<E>> pq //it is not initialized thats why NPE occurs.

to

private PriorityQueue<MyNode<E>> pq = new PriorityQueue<MyNode<E>>();

No need to have a ComparatorMyNode<E> as a separate class. Instead of this, you can implements Comparable like MyNode<E> implements Comparable<E> in your MyNode class and can override the

public int compareTo(E o) { return 0; }

method in your MyNode class.