import java.util.*;
//Implementation of a Queue (First In First Out List) of Objects
public class Queue
{
   //Instance Variable: Stores the Objects added into this Stack
   private LinkedList<Object> store;
   
   public Queue()
   {
      this.store = new LinkedList<Object>();
   }
   
   
   //Answers true iff this Queue is empty
   public boolean isEmpty()
   {
      return this.store.isEmpty();
   }
   
   //Accessor: Returns the Object at the HEAD without removing it
   //          Returns null if this Queue is empty
   public Object peek()
   {
      if (this.store.isEmpty())
         return null;
      return this.store.getFirst();
   }
   
   //Mutator: Adds an Object to the REAR of this Queue
   public void enQueue(Object item)
   {
      this.store.addLast(item);
   }
   
   //Mutator: Removes and returns the Object from the HEAD of this Queue
   //         Throws a "Queue Underflow" Exception if this Queue is empty
   public Object deQueue()
   {
      if (this.store.isEmpty())
         throw new RuntimeException("Queue Underflow");
      return this.store.removeFirst();
   }
}