import ListNode;

public class Queue {

  private ListNode front;                // first node in list
  private ListNode tail;                 // last node in list
  private int length = 0;                // queue length


  /**
   * enqueue
   * 
   * insert a new node into the tail if the list
   */
  public void enqueue( Object element ) {

    ListNode node = new ListNode();
    node.setElement( element );

    if ( isEmpty() ) {
       front = node;
       tail = node;
    }
    else {
       tail.setNext( node );
       tail = node;
    }

    length++;
  }

  /** 
   * dequeue
   *
   * remove and return the item at the front of the list
   */
  public Object dequeue() {

    if ( isEmpty() ) {
       return null;
    }
    else {
       length--;
       ListNode frontNode = front;
       front = front.getNext();
       return frontNode.getElement();
    }

  }

  /**
   * remove
   *
   * remove current node from list
   */
  public void remove( ListNode current ) {
  
    if ( current == front ) {
       front = current.getNext();
    }
    else {
 
      // find the node before current
      ListNode node = front;

      while( current != node.getNext() ) {
           node = node.getNext();
      }
 
      node.setNext( current.getNext() );

      if ( current == tail ) {
          tail = node;
      }
    }

    length--;
  }

  /**
   * isEmpty
   *
   * return true if list is empty
   */
  public boolean isEmpty() {
       
    return front == null;
  }

  /**
   * notEmpty
   *
   * return true if list is not empty
   */
  public boolean notEmpty() {

     return front != null;
  }

  /**
   * getFront
   * 
   * return the front node
   */
  public ListNode getFront() {

     return front;
  }

  /**
   * setFront
   *
   * place node at front of the list
   */
  private void setFront( ListNode n ) {
    
    if ( isEmpty() ) {
        enqueue( n );
    }
    else {
        n.setNext( front );
        front = n;
    }
  }      
 
   
  /**
   * enqueueFront
   * 
   * place node at front the list
   */
  public void enqueueFront( Object element ) {

     ListNode n = new ListNode();
     n.setElement( element );

     setFront( n );
  } 

  /**
   * getTail
   *
   * return the last node in the list
   */
  public ListNode getTail() {
    
     return tail;
  }

  /**
   * getFrontElement
   *
   * return the reference to the object in the first node
   */
  public Object getFrontElement() {

    return( front.getElement() );
  }

  /**
   * getQueueLength
   */
  int getQueueLength() {
    return length;
  }
}