Creating a list of assignments for a school project, but I'm getting a NullPointerException

Question

I'm unsure whether or not this has been answered before as I didn't see it under Similar Questions but here goes... For a school project, we have to create a doubly-linked list of Assignment s and when I made my addAssignment(Assignment a) function, I got a NullPointerException.

In our project, Assignment s consist of a String for the name of the assignment, int s for the number of points earned and the number of points given, a double for the percentage grade and a char for the letter grade given. Node s contain an Assignment called "assignment" and "next" and "prev" Node references. A DList (our doubly-linked list) consists of a "head" and "tail" Node at the start.

The code in question is included below. I've included comments detailing where my console gave me error messages.

In class Assignment :

/**
 * setter
 * @param n = the name of the assignment
 */
public void setName(String n) {
    name = n;
}

In class Node :

/**
 * default constructor
 */
public Node() {
    assignment.setName("");
    assignment.setEarnedPts(0);
    assignment.setTotalPts(0);
    next = prev = null;
}

/**
 * n-argument constructor
 * @param a = assignment
 * @param e = # of earned points
 * @param t = # of total possible points
 * @param n = "next" node
 * @param p = "previous" node
 */
public Node(String name, int e, int t, Node n, Node p) {
    assignment.setName(name); // got an error here
    assignment.setEarnedPts(e);
    assignment.setTotalPts(t);
    assignment.setGrade();
    next = n;
    prev = p;
}

Finally, in class DList :

/**
 * adds an assignment to the list
 * @param a = the assignment
 */
public void addAssignment(Assignment a) {
    // check to see if this is the first node in the list
    Node temp = new Node(a.getName(), a.getEarnedPts(), a.getTotalPts(), tail, head); // got an error here
    if (head == tail) {
        head = temp;
    }
    else { // there are other node(s) in the list
        Node p = head;
        Node q = new Node();
        while (p.getNext() != null) {
            q = p; // set q equal to p, so that q will equal previous node
            p = p.getNext(); // advance p by one node
        }
        p.setNext(temp);
        p.setPrev(q); // ??? ask about this in class later
    }
}

Answer 1

Unfortunately we don't have the entire code, but I suspect, that in your class Node you have a field like this:

class Node{
    Assignment assignment;

If that's the case this is your problem: You did not initialize any value to the assignment variable and it is therefore Null. Java does not default initialize like, for example, C++ does. You will have to do it yourself, the constructors for the Node class then look like this:

public Node() {
    assignment = new Assignment();
    assignment.setName("");
    assignment.setEarnedPts(0);
    assignment.setTotalPts(0);
    next = prev = null;
}

public Node(String name, int e, int t, Node n, Node p) {
    assignment = new Assignment();
    assignment.setName(name); // got an error here
    assignment.setEarnedPts(e);
    assignment.setTotalPts(t);
    assignment.setGrade();
    next = n;
    prev = p;
}