import java.util.NoSuchElementException;

/** * A classic first-in first-out queue data structure implemented * as a linked sequence of nodes. * * @param * the type of element held in this queue */

public class Queue {

/** * Queue uses a node based implementation, where the nodes form a singly * linked-list like structure. The queue must maintain a reference to the * node holding the element at the front of the queue, and a reference to * the node holding the element at the end of the queue. */

/** * The node at the front of the queue (the first node, or head, of the * linked list). */ private Node front;

/** * The node at the back of the queue (the last node of the linked list) */ private Node back;

/** * The number of elements stored in this queue (or the number of nodes) */ private int size;

/** * Creates an empty queue (size is zero). */ public Queue() { } /** * Enqueue an element. This operation adds the element to the back of the * queue. * * @param element * the element to add to the back of the queue */ public void enqueue(E element) { }

/** * Dequeue an element. This operation removes and returns the element at the * front of the queue if the queue is not empty. * * @return the element at the front of the queue if the queue is not empty * @throws NoSuchElementException * if the queue is empty */ public E dequeue() { }

/** * Return the element at the front of the queue without dequeuing the * element. * * @return the element at the front of the queue if the queue is not empty * @throws NoSuchElementException * if the queue is empty */ public E peek() { }

/** * Returns the number of elements in this queue. * * @return the number of elements in this queue */ public int size() { }

/** * Returns true if this queue is empty, and false otherwise. * * @return true if this queue is empty, and false otherwise */ public boolean isEmpty() { }

/** * Returns the node at the front of the queue. This method is here for * debugging and testing purposes. * * @return the node at the front of the queue */ Node getFront() { return this.front; }

/** * Returns the node at the back of the queue. This method is here for * debugging and testing purposes. * * @return the node at the back of the queue */ Node getBack() { return this.back; }

/** * Returns a hash code for this queue. The hash code is computed using the * elements of this stack. * * @return a hash code for this queue */ @Override public int hashCode() { final int prime = 31; int result = 1; Node n = this.front; while (n != null) { result = prime * result + n.getElement().hashCode(); n = n.getNext(); } return result; }

/** * Compares the specified object to this queue for equality. Two queues are * equal if they contain the same number of elements in the same order. * * @param obj * the object to compare to this queue for equality * @return true if the specified object is equal to this queue */ @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (this.getClass() != obj.getClass()) { return false; } Queue other = (Queue) obj; if (this.size == other.size()) { Node n = this.front; Node nOther = other.front; for (int i = 0; i < this.size; i++) { if (!n.getElement().equals(nOther.getElement())) { return false; } } }

return true; }

/** * Returns a string representation of this queue. The string representation * consists of a list of the queue's elements from the front of the queue to * the back of the queue, enclosed in square brackets ("[]"). Adjacent * elements are separated by the characters ", " (comma and space). Elements * are converted to strings as by String.valueOf(Object). * * @return a string representation of this queue */ @Override public String toString() { // see equals for an example of iterating over the nodes of the queue } }