For Queue<E>, we are using the LinkedList<E> implementation and we are
using the following operations:

 public void add(E value);  // inserts given value at the end of the queue
 public E remove();         // removes and returns the front of the queue
 public E peek();           // returns the value at the front of the queue without removing
 public boolean isEmpty();  // returns whether or not queue is empty
 public int size();         // returns number of elements in the queue