Why does the list appear empty while trying to add nodes in the beginning?

Question

I've literally lost my mind trying to figure out why my add() and print() methods won't work. I've tried virtually everything but I just can't do this. I know my code is dead wrong (I can't even tell if my code was right at one point or another because I deleted it to try new things) so what could be wrong with it?

I appreciate you taking the time to read.

NodeFN class:

public class NodeFN {
     private String data; // Data for node.
     private NodeFN next; // Next node.

public NodeFN(String data) {
    this.data = data; // Take the data value passed in & store it in the data field.
    this.next = null; // Take the next node & store it in the next field.
}

   // Mutator functions.
public String getData() {return data;}
public NodeFN getNext() {return next;}
public void setData(String d) {data = d;}
public void setNext(NodeFN n) {next = n;}
} 

Queue class:

public class Queue {
   NodeFN head; // Head of node.
   public String n;

public Queue(String n) { 
    head = new NodeFN(n); // head is now an object of NodeFN which holds a string.
}

public void add(String n) {
    NodeFN nn = new NodeFN(n); // nn is now an object of NodeFN which holds a string, it should return something.
        if(head == null) {
            head = nn;
        }
        while(nn.getData().compareTo(head.getData()) < 0) {
                nn.setNext(head); // Put node in beginning of the list.
                nn.setData(n);      
        }
    }

public void print() {
    NodeFN nn = new NodeFN(n);

    while(nn != null) {
        nn.getNext().getData();
        System.out.println(nn.getData() + " ");
    }
}

public static void main(String[] args) {
    Queue q = new Queue("string to test");
    q.add("another string to test if add method works.");
    q.print();
}
}

Answer 1

You're creating a linked list. In order to print that, you need to loop through the list until you reach a null in getNext() . Essentially, you should end up with something like this:

public void print() {
   NodeFN current = head;

   while(current != null) {
       System.out.println(current.getData());
       current = current.getNext();
   }
}

As for your add() method, the idea is to basically put any new nodes as the next reference of the last node in your list. Simply keep a reference to the last node in your Queue class. When adding, set next of the last node to your newly created node, and set the new node as the last .

Answer 2

I can't speak for your add method, but what is n here ?

public void print() {
    NodeFN nn = new NodeFN(n);

    while(nn != null) {
        nn.getNext().getData();
        System.out.println(nn.getData() + " ");
    }
}

The queue class should not care about public String n at all. You only need the head Node.

Then, nn.getNext().getData(); is returning something, yes? But, you are not printing it nor are you "moving forward" in the list. (You don't assign nn to the next node).

Try something like this

public void print() {
    if (head == null) System.out.println("()");

    NodeFN tmp = head;

    while(tmp != null) {
        System.out.println(tmp.getData() + " ");
        tmp = tmp.getNext();
    }
}

---

If you want the nodes to be add at the start of the list, then this should work.

public void add(String n) {
    NodeFN nn = new NodeFN(n);
    if(head == null) {
        head = nn;
    }

    // Don't use a while loop, there is nothing to repeat
    if (n.compareTo(head.getData()) < 0) {
        // Both these operations put 'nn' in beginning of the list.
        nn.setNext(head); 
        head = nn;
    }
}

Answer 3

For the add method, you had the right idea to start. Make nn the head if there are no element in the list yet. Otherwise, traverse the list to the last element and add it to the end (since it's a queue).

public void add(String n) {
    NodeFN nn = new NodeFN(n);
    if(head == null) {
        head = nn;
    }
    else {
        NodeFN cursor = this.head;
        while(cursor.getNext() != null) {
            cursor = cursor.getNext();
        }
        cursor.setNext(nn);
    }
}

If you want to add the new node to the beginning (for whatever strange reason), it would be even easier.

public void add(String n) {
    NodeFN nn = new NodeFN(n);
    nn.setNext(this.head);
    this.head = nn;
}

For the print method. You're not setting nn (the cursor) to reference any node in the actual queue. You should be setting nn to the head of the queue and then iterating through the queue. NodeFN nn = this.head . And then in the while loop's body, print the data nn.getData() , then move to the next node nn = nn.getNext() .

public void print() {
    NodeFN cursor= this.head;

    while(cursor != null) {
        System.out.println(cursor.getData() + " ");
        cursor= cursor.getNext();           
    }
}