//Implementation of a Stack (Last In First Out List) of Objects
import java.util.*;
public class Stack
{
   //Instance Variable: Stores the Objects added into this Stack
   private LinkedList<Object> store;
   
   //Constructor: Initializes an empty Stack
   public Stack()
   {
      this.store = new LinkedList<Object>();
   }
   
   //Accessor: Answers whether this Stack is empty
   public boolean isEmpty()
   {
      return this.store.isEmpty();
   }
   
   //Accessor: Returns the Object on the TOP without removing it
   //          Returns null if this Stack is empty
   public Object peek()
   {
      if (this.store.isEmpty())
         return null;
      return this.store.getFirst();
   }
   
   //Mutator: Adds an Object to the TOP of this Stack
   public void push(Object item)
   {
      this.store.addFirst(item);
   }
   
   //Mutator: Removes and returns the Object from the TOP of this Stack
   //         Throws a "Stack Underflow" Exception if this Stack is empty
   public Object pop()
   {
      if (this.store.isEmpty())
         throw new RuntimeException("Stack Underflow");
      return this.store.removeFirst();
   }   
}