public class QueueUsingLinkedList {
  private Node front, rear;
  private int currentSize;

  private class Node {
    int data;
    Node next;
  }

  public QueueUsingLinkedList() {
    front = null;
    rear = null;
    currentSize = 0;
  }

  public boolean isEmpty() {
    return (currentSize == 0);
  }

  //Remove item from the beginning of the list.
  public int dequeue() {
    int data = front.data;
    front = front.next;
    if (isEmpty()) {
      rear = null;
    }
    currentSize--;
    System.out.println(data + " removed from the queue");
    return data;
  }

  //Add data to the end of the list.
  public void enqueue(int data) {
    Node oldRear = rear;
    rear = new Node();
    rear.data = data;
    rear.next = null;
    if (isEmpty()) {
      front = rear;
    } else {
      oldRear.next = rear;
    }
    currentSize++;
    System.out.println(data + " added to the queue");
  }

  public static void main(String a[]) {

    QueueUsingLinkedList queue = new QueueUsingLinkedList();
    queue.enqueue(6);
    queue.dequeue();
    queue.enqueue(3);
    queue.enqueue(9);
    queue.enqueue(5);
    queue.dequeue();
    queue.enqueue(4);
    queue.dequeue();
    queue.enqueue(8);
    queue.enqueue(7);
    queue.dequeue();
    queue.enqueue(2);
    queue.enqueue(1);
  }
}