// Stack.java: a stack implementation based on linked lists

public class Stack<E> {
   private java.util.LinkedList<E> items = new java.util.LinkedList<>();

   public void push(E newItem) {
      items.add(newItem);
   }

   public E pop() {
      return items.remove(items.size() - 1);
   }

   public boolean isEmpty() {
      return items.isEmpty();
   }

   public static void main(String[] args) {
      Stack<Integer> nums = new Stack<>();
      if ( !nums.isEmpty() )
         System.err.println("Error: initial stack not empty.");
      for(int i = 0; i < 20; ++i)
         nums.push(i);
      if ( nums.isEmpty() )
         System.err.println("Error: stack still empty.");
      for(int i = 19; i >= 0; --i)
         if ( nums.pop() != i )
            System.err.println("Error: pop of " + i + " out of order.");
      if ( !nums.isEmpty() )
         System.err.println("Error: final stack not empty.");
      System.out.println("All tests completed.");
   }
}