// Queue.java: a queue implementation based on linked lists

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

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

   public E remove() {
      return items.remove(0);
   }

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

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