LibTest/collection/DoubleLinkedQueue/firstEntry_A02_t01.dart

// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.9

/// @assertion DoubleLinkedQueueEntry<E> firstEntry()
/// @description Checks that null is returned if this queue is empty.
/// @note undocumented
/// @author kaigorodov

import "../../../Utils/expect.dart";
import "dart:collection";

main() {
  DoubleLinkedQueue queue = new DoubleLinkedQueue();
  
  Expect.isNull(queue.firstEntry());
  
  queue.addFirst(1);
  queue.addFirst(2);
  queue.clear();
  
  Expect.isNull(queue.firstEntry());
}