* already a {@code FluentIterable}.
   *
   * <p><b>{@code Stream} equivalent:</b> {@link Collection#stream} if {@code iterable} is a {@link
   * Collection}; {@link Streams#stream(Iterable)} otherwise.
   */
  public static <E extends @Nullable Object> FluentIterable<E> from(Iterable<E> iterable) {
    return (iterable instanceof FluentIterable)
        ? (FluentIterable<E>) iterable

   * already a {@code FluentIterable}.
   *
   * <p><b>{@code Stream} equivalent:</b> {@link Collection#stream} if {@code iterable} is a {@link
   * Collection}; {@link Streams#stream(Iterable)} otherwise.
   */
  public static <E extends @Nullable Object> FluentIterable<E> from(Iterable<E> iterable) {
    return (iterable instanceof FluentIterable)
        ? (FluentIterable<E>) iterable

    FluentIterable<TimeUnit> unused = FluentIterable.from(TimeUnit.values()).append(SECONDS);
  }

  public void testFromArrayAndIteratorRemove() {
    FluentIterable<TimeUnit> units = FluentIterable.from(TimeUnit.values());
    assertThrows(UnsupportedOperationException.class, () -> removeIf(units, equalTo(SECONDS)));
  }

  public void testFrom() {
    assertEquals(
        ImmutableList.of(1, 2, 3, 4),

   *
   * @deprecated Use {@link com.google.common.graph.Traverser#depthFirstPreOrder} instead, which has
   *     the same behavior.
   */
  @Deprecated
  public final FluentIterable<T> preOrderTraversal(T root) {
    checkNotNull(root);
    return new FluentIterable<T>() {
      @Override
      public UnmodifiableIterator<T> iterator() {
        return preOrderIterator(root);
      }
    };
  }

    assertThat(onlyPresent).containsExactly(2);
  }

  private static Optional<Integer> getSomeOptionalInt() {
    return Optional.of(1);
  }

  private static FluentIterable<? extends Number> getSomeNumbers() {
    return FluentIterable.from(ImmutableList.<Number>of());
  }

  /*
   * The following tests demonstrate the shortcomings of or() and test that the casting workaround

  public static <E> Iterable<E> unmodifiableIterable(ImmutableCollection<E> iterable) {
    return checkNotNull(iterable);
  }

  private static final class UnmodifiableIterable<T extends @Nullable Object>
      extends FluentIterable<T> {
    private final Iterable<? extends T> iterable;

    private UnmodifiableIterable(Iterable<? extends T> iterable) {
      this.iterable = iterable;
    }

    @Override

  public static <E> Iterable<E> unmodifiableIterable(ImmutableCollection<E> iterable) {
    return checkNotNull(iterable);
  }

  private static final class UnmodifiableIterable<T extends @Nullable Object>
      extends FluentIterable<T> {
    private final Iterable<? extends T> iterable;

    private UnmodifiableIterable(Iterable<? extends T> iterable) {
      this.iterable = iterable;
    }

    @Override

  public void testConstants_areUnique() {
    assertThat(getConstants()).containsNoDuplicates();
  }

  @J2ktIncompatible
  @GwtIncompatible // reflection
  private static FluentIterable<Field> getConstantFields() {
    return FluentIterable.from(asList(MediaType.class.getDeclaredFields()))
        .filter(
            field -> {
              int modifiers = field.getModifiers();
              return isPublic(modifiers)

   *
   * {@snippet :
   * Optional<Integer> optionalInt = getSomeOptionalInt();
   * Number value = optionalInt.or(0.5); // error
   *
   * FluentIterable<? extends Number> numbers = getSomeNumbers();
   * Optional<? extends Number> first = numbers.first();
   * Number value = first.or(0.5); // error
   * }
   *

   *
   * {@snippet :
   * Optional<Integer> optionalInt = getSomeOptionalInt();
   * Number value = optionalInt.or(0.5); // error
   *
   * FluentIterable<? extends Number> numbers = getSomeNumbers();
   * Optional<? extends Number> first = numbers.first();
   * Number value = first.or(0.5); // error
   * }
   *