*     iterator is unspecified.
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(
      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
    return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
  }

  /**
   * Copies an iterator's elements into an array. The iterator will be left exhausted: its {@code

   *     iterator is unspecified.
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(
      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
    return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
  }

  /**
   * Copies an iterator's elements into an array. The iterator will be left exhausted: its {@code

import static com.google.common.base.Preconditions.checkElementIndex;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.Iterables.getOnlyElement;
import static com.google.common.collect.Iterators.emptyIterator;
import static com.google.common.collect.Iterators.peekingIterator;
import static com.google.common.collect.Range.rangeLexOrdering;

 * limitations under the License.
 */

package com.google.common.io;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.Iterables.getOnlyElement;
import static java.nio.file.LinkOption.NOFOLLOW_LINKS;
import static java.util.Objects.requireNonNull;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.J2ktIncompatible;

  static <E> ImmutableSet<E> asImmutable(Set<E> delegate) {
    switch (delegate.size()) {
      case 0:
        return ImmutableSet.of();
      case 1:
        return ImmutableSet.of(Iterables.getOnlyElement(delegate));
      default:
        return new ImmutableEnumSet<E>(delegate);
    }
  }

  public ImmutableEnumSet(Set<E> delegate) {
    super(delegate);
  }

    interfaces.remove(AutoCloseable.class);
    @SuppressWarnings("unchecked") // super interface type of T
    Class<? super T> interfaceType = (Class<? super T>) Iterables.getOnlyElement(interfaces);
    new ForwardingWrapperTester()
        .testForwarding(
            interfaceType,
            delegate -> {
              T mock = mock(forwarderClass, CALLS_REAL_METHODS);
              try {

    interfaces.remove(AutoCloseable.class);
    @SuppressWarnings("unchecked") // super interface type of T
    Class<? super T> interfaceType = (Class<? super T>) Iterables.getOnlyElement(interfaces);
    new ForwardingWrapperTester()
        .testForwarding(
            interfaceType,
            delegate -> {
              T mock = mock(forwarderClass, CALLS_REAL_METHODS);
              try {

    service.stopAsync();
    assertEquals(State.TERMINATED, service.state());

    assertThrows(IllegalStateException.class, service::startAsync);
    assertEquals(State.TERMINATED, Iterables.getOnlyElement(listener.getStateHistory()));
  }

  public void testFailingServiceStartAndWait() throws Exception {
    StartFailingService service = new StartFailingService();

    service.stopAsync();
    assertEquals(State.TERMINATED, service.state());

    assertThrows(IllegalStateException.class, service::startAsync);
    assertEquals(State.TERMINATED, Iterables.getOnlyElement(listener.getStateHistory()));
  }

  public void testFailingServiceStartAndWait() throws Exception {
    StartFailingService service = new StartFailingService();

      assertThat(future.isDone()).isTrue();
      // inspect state and ensure it is correct!
      // asserts that all get calling threads received the same value
      Object result = Iterables.getOnlyElement(finalResults);
      if (result == CancellationException.class) {
        assertTrue(future.isCancelled());
        if (future.wasInterrupted()) {