Working with a toString and a queue in Java

Question

I've switched everything around within my toString to try and get the result I wanted, but I'm having no luck. I'm trying to get this result:

QUEUE TESTING

3

7

7

4

The size of the queue is: 3

The queue contains:

4 9 8

But instead, I'm getting this:

QUEUE TESTING
3
7
7
4
The size of the queue is: 3
The queue contains:
 9 8

And I can't understand why the four isn't being returned with the 9 and 8. Can someone please tell me whats going on. Thanks!

Here's the code I'm working with (attached files left-out):

public class Murray_A06Q1 {

    public static void main(String[] args) {

        LinkedQueue<Integer> queue = new LinkedQueue<Integer>();
        System.out.println("QUEUE TESTING");

        queue.enqueue(3);
        queue.enqueue(7);
        queue.enqueue(4);
        System.out.println(queue.first());
        queue.dequeue();        
        queue.enqueue(9);
        queue.enqueue(8);
        System.out.println(queue.first());        
        System.out.println(queue.dequeue());
        System.out.println(queue.first());        

        System.out.println("The size of the queue is: " + queue.size());
        System.out.println("The queue contains:\n" + queue.toString());        
    }


  public static class LinkedQueue<T> implements QueueADT<T> {
    private int count;
    private LinearNode<T> head, tail; //front, back

    // Constructor      
    public LinkedQueue() {
        count = 0;
        head = tail = null;
    }

    // Adds the specified element to the tail of this queue.   
    public void enqueue(T element) {

        LinearNode<T> node = new LinearNode<T>(element);

        if (isEmpty())
            head = node;
        else
            tail.setNext(node);

        tail = node;
        count++;
    }

    //Removes the element at the head of this queue and returns a
    public T dequeue() throws EmptyCollectionException {
        if (isEmpty())
            throw new EmptyCollectionException("queue");

        T result = head.getElement();
        head = head.getNext();
        count--;

        if (isEmpty())
            tail = null;

        return result;
    } // End of dequeue

    // first() throws EmptyCollectionException         
    public T first() throws EmptyCollectionException {
        return head.getElement();
    }

    // Beginning of isEmpty()           
    public boolean isEmpty() {
        if(count == 0){
            return true;
        }   
        else {
            return false;           
        }
    } // End of isEmpty()

    // Beginning of size()          
    public int size() {
        return count;
    }

    // Begin of toString() method           
    public String toString() {

        if (isEmpty()) {
            return " ";
        }

        StringBuilder sb = new StringBuilder();
        LinearNode<T> next = head.getNext();

        while(next != null){
            sb.append(" ").append(next.getElement());
            next = next.getNext();
        }
        return sb.toString();   
    } // End of toString method

  }

}

Is it because you are starting at the node head.getNext(); instead of starting with the node head? — gla3dr
– gla3dr, Commented Aug 4, 2015 at 20:35

rgettman · Accepted Answer · 2015-08-04 20:35:45Z

2

You are starting with the second element. In toString(), you initialize next with head.getNext(), where head is the first and head.next() is the second element. This causes the output to skip the first element, and this would cause a NullPointerException if there were exactly one element in the queue.

Initialize next to head instead.

answered Aug 4, 2015 at 20:35

rgettman

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Comments

Sami Awwad · Accepted Answer · 2015-08-04 20:41:14Z

1

You need to write your toString method like this:

public String toString() {

    if (isEmpty()) {
        return " ";
    }

    StringBuilder sb = new StringBuilder();
    LinearNode<T> next = head;

    while(next != null){
        sb.append(" ").append(next.getElement());
        next = next.getNext();
    }

    return sb.toString();

} // End of toString method

toString was never printing the HEAD value since you assigned head.getNext to next before your while loop.

answered Aug 4, 2015 at 20:41

Sami Awwad

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