* @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;
  }

public final class Iterators {
  private Iterators() {}

  /**
   * Returns the empty iterator.
   *
   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
   */
  static <T extends @Nullable Object> UnmodifiableIterator<T> emptyIterator() {
    return emptyListIterator();
  }

  /**
   * Returns the empty iterator.
   *

   *     // operate on tuple
   *   }
   * }
   * }</pre>
   *
   * <p>Note that if any input set is empty, the Cartesian product will also be empty. If no sets at
   * all are provided (an empty list), the resulting Cartesian product has one element, an empty
   * list (counter-intuitive, but mathematically consistent).
   *

                        EMPTY,
                        request);
            }

            validateRawRepositories(problems, m.getRepositories(), "repositories.repository.", EMPTY, request);

            validateRawRepositories(
                    problems, m.getPluginRepositories(), "pluginRepositories.pluginRepository.", EMPTY, request);

            Build build = m.getBuild();

  }

  private static final ImmutableMap<Class<?>, Method> EMPTY_GENERATORS;

  static {
    ImmutableMap.Builder<Class<?>, Method> builder = ImmutableMap.builder();
    for (Method method : FreshValueGenerator.class.getDeclaredMethods()) {
      if (method.isAnnotationPresent(Empty.class)) {
        builder.put(method.getReturnType(), method);
      }
    }
    EMPTY_GENERATORS = builder.buildOrThrow();
  }

			expectedErr: errInvalidRuleID,
		},
		{ // Rule with empty ID
			inputXML: `<Rule>
							<ID></ID>
							<Filter><Prefix></Prefix></Filter>
							<Expiration>
								<Days>365</Days>
							</Expiration>
                            <Status>Enabled</Status>
	                    </Rule>`,
			expectedErr: nil,
		},
		{ // Rule with empty status
			inputXML: ` <Rule>

		if bucket == "" {
			return errors.New("bucket name cannot be empty")
		}

		return clnt.RemoveObject(context.Background(), bucket, object, minio.RemoveObjectOptions{})

	case "Mkdir":
		bucket, prefix := path2BucketObject(r.Filepath)
		if bucket == "" {
			return errors.New("bucket name cannot be empty")
		}

		if prefix == "" {

  }

  /**
   * Returns an empty immutable range map.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings("unchecked")
  public static <K extends Comparable<?>, V> ImmutableRangeMap<K, V> of() {
    return (ImmutableRangeMap<K, V>) EMPTY;
  }

  /** Returns an immutable range map mapping a single range to a single value. */

	// ErrRegistered registered
	ErrRegistered = errors.New("registered")
	// ErrInvalidField invalid field
	ErrInvalidField = errors.New("invalid field")
	// ErrEmptySlice empty slice found
	ErrEmptySlice = errors.New("empty slice found")
	// ErrDryRunModeUnsupported dry run mode unsupported
	ErrDryRunModeUnsupported = errors.New("dry run mode unsupported")
	// ErrInvalidDB invalid db
	ErrInvalidDB = errors.New("invalid db")

	d, err := config.getDriver(ctx, tier)
	if err != nil {
		return err
	}
	return checkWarmBackend(ctx, d)
}

// Empty returns if tier targets are empty
func (config *TierConfigMgr) Empty() bool {
	if config == nil {
		return true
	}
	return len(config.ListTiers()) == 0
}

// TierType returns the type of tier