Java - Queues as LinkedLists - Adding new nodes to the Queue

Question

Im working with a Queue as a LinkedList adding elements to the back of the list and removing them from the front. However I am struggling with the concept of adding new nodes as I cant manage to link the pre-existing nodes together using FIFO.

You can see my method below this picture.

enter image description here

 public void add(Object data)
    {
 if data == null) throw new IllegalArgumentException() ;

 Node newNode = new Node (data, null);

 if (size == 0){ //- remember we are adding elements to the back
   tail = newNode;
   head = newNode;
 }
 else {
  tail = newNode; //right
  newNode.next = head.next; //??? What should be in place of this?

 }
 size++;

    }

Im following the diagram but dont know how to reference the previous node.Here is the information given to the class.

public class QueueNode
{
    private Node head, tail ;
    private int size ;
    /**
       inner class for Node
    */
    private static class Node
    {
 Node next ;
 Object data ;

 public Node(Object data, Node n)
 {
     next = n ;
     this.data = data ;
 }
    }
    /**
       Constructor for queue
     */
    public QueueNode()
    {
 head = tail = null ;
 size = 0 ;
    }

//rest of class (not needed)

Why do you need to reference the previous node? If you really do, then maybe a doubly linked list is what you need. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Mar 17, 2014 at 0:01
When I add a new node and make it the tail I don't know how to link it to the node above it. I thought head.next would work but it doesn't. — Jebathon
– Jebathon, Commented Mar 17, 2014 at 0:04

Raul Guiu · Accepted Answer · 2014-03-17 00:11:57Z

1

You will only need to set the next of the current tail ad the node being added. And the newnode will became the tail.

tail.next = newNode;
tail = newNode

You tried:

tail = newNode; //right

// yes, right, but set the 'next' of the current tail first, before you lose the reference.

newNode.next = head.next; //??? What should be in place of this?

// No, you dont need to do anything with the head when adding new nodes.

answered Mar 17, 2014 at 0:11

Raul Guiu

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