this is for an assignment. I need to implement a queue using linked lists. I keep getting a null pointer exception while using my enqueue method for my listQueue Class. I'm not too sure what is going on. Below are my main class and listQueue Class.
public class Driver{
public static void main(String[] args) {
MyQueue<Integer> q = null;
boolean useList = true; // make it false if you want to array implementation
if (useList)
q = new ListQueue<Integer>(); // default setting: use a list to implement the queue. You can change it.
else
q = new ArrayQueue<Integer>();
for(int i = 0; i < 1000; i++) // add a large number of items onto the queue
{
q.enqueue(i);
}
System.out.println("Now, dequeue items!");
while(!q.isEmpty())
{
System.out.print(q.dequeue() + " ");
}
System.out.println("\nEnd of dequeueing");
// you should fully test/debug all methods!
}
}
public class ListQueue<T> implements MyQueue<T> {
private class Node {
T item; // public
Node next; // null
}
private Node head = null;
private Node tail = null;
private int size = 0; //size of the list
public ListQueue(){}
/**
* is empty?
*/
public boolean isEmpty() {
// your implementation here:
if (size == 0) {
return true;
}//end if
return false;
}//end isEmpty
/**
* @return the number of items in the queue
*/
public int size() {
// your implementation here:
return size;
}//end size
/**
* enqueue: add an element at the tail of queue
*/
public void enqueue(T item) {
// your implementation here:
if (size == 0) {
head = new Node();
head.item = item;
head.next=tail;
size++;
}//end if
else{
Node newNode = new Node();
newNode.item=item;
tail.next = newNode;
tail = newNode;
size++;
}//end else
}
/**
* dequeue: remove and return the head of the queue
*
* @return the deleted value
* @throws NoSuchElementException if queue is empty
*/
public T dequeue() {
// your implementation here:
if(isEmpty()){
throw new NoSuchElementException();
}//end if
else{
Node temp = head;
head = head.next;
size--;
return temp.item;
}//end else
}//end dequeue
/**
* peek: view the head of queue without removing it.
*
* @return Null if queue is empty
*/
public T peek() {
// your implementation here:
if(isEmpty()){
return null;
}//end if
else{
return head.item;
}//end else
}//end peek
}//end ListQueue
