verifySetContents(set, EMPTY_COLLECTION);
  }

  public void testNewTreeSetEmpty() {
    TreeSet<Integer> set = Sets.newTreeSet();
    verifySortedSetContents(set, EMPTY_COLLECTION, null);
  }

  public void testNewTreeSetEmptyDerived() {
    TreeSet<Derived> set = Sets.newTreeSet();
    assertTrue(set.isEmpty());
    set.add(new Derived("foo"));
    set.add(new Derived("bar"));

    assertTrue(map.isEmpty());
    assertNull(map.get("five"));
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

  public void testAsMapSorted() {
    SortedSet<String> strings = new NonNavigableSortedSet();

    assertTrue(map.isEmpty());
    assertNull(map.get("five"));
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

  public void testAsMapSorted() {
    SortedSet<String> strings = new NonNavigableSortedSet();

     *
     * @param <E>
     *            {@link TreeSet}の要素型
     * @return {@link TreeSet}の新しいインスタンス
     * @see TreeSet#TreeSet()
     */
    public static <E> TreeSet<E> newTreeSet() {
        return new TreeSet<>();
    }

    /**
     * {@link TreeSet}の新しいインスタンスを作成して返します。
     *
     * @param <E>
     *            {@link TreeSet}の要素型
     * @param c

   */
  @SuppressWarnings("rawtypes") // https://github.com/google/guava/issues/989
  public static <E extends Comparable> TreeSet<E> newTreeSet(Iterable<? extends E> elements) {
    TreeSet<E> set = newTreeSet();
    Iterables.addAll(set, elements);
    return set;
  }

  /**
   * Creates a <i>mutable</i>, empty {@code TreeSet} instance with the given comparator.
   *