FluentIterable<? extends Number> numbers = getSomeNumbers();

    // Sadly, the following is what users will have to do in some circumstances.

    @SuppressWarnings("unchecked") // safe covariant cast
    Optional<Number> first = (Optional<Number>) numbers.first();
    Number value = first.or(0.5); // fine
  }

  @J2ktIncompatible
  @GwtIncompatible // NullPointerTester

 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code
 * Optional}</a>.
 *
 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
 *     this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
 *     supertype {@code S} of {@code T}.
 * @author Kurt Alfred Kluever
 * @author Kevin Bourrillion
 * @since 10.0
 */

    // Count how long it actually took to return; we'll accept any number between the expected delay
    // and the approximate actual delay, to be robust to variance in thread scheduling.
    char overWaitNanosFirstDigit =
        Long.toString(
                thread.timeSpentBlocked - TimeUnit.MILLISECONDS.toNanos(longWait ? 5000 : 3000))
            .charAt(0);

   * either the {@code x} or {@code y} data must have a non-zero population variance (i.e. {@code
   * xStats().populationVariance() > 0.0 || yStats().populationVariance() > 0.0}). The result is
   * guaranteed to be horizontal if there is variance in the {@code x} data but not the {@code y}
   * data, and vertical if there is variance in the {@code y} data but not the {@code x} data.
   *

  protected Entry<K, V> entry(K key, V value) {
    return Helpers.mapEntry(key, value);
  }

  @Override
  protected void expectContents(Collection<Entry<K, V>> expected) {
    // TODO: move this to invariant checks once the appropriate hook exists?
    super.expectContents(expected);
    for (Entry<K, V> entry : expected) {
      assertEquals(

   *
   * <p>Normally this method leaves the elements at up to {@code index - 1}, inclusive, untouched.
   * Under these circumstances, it returns {@code null}.
   *
   * <p>Occasionally, in order to maintain the heap invariant, it must swap a later element of the
   * list with one before {@code index}. Under these circumstances it returns a pair of elements as

        checkNotNull(cell.getRowKey(), "row");
        checkNotNull(cell.getColumnKey(), "column");
        checkNotNull(cell.getValue(), "value");
        @SuppressWarnings("unchecked") // all supported methods are covariant
        Cell<R, C, V> immutableCell = (Cell<R, C, V>) cell;
        cells.add(immutableCell);
      } else {
        put(cell.getRowKey(), cell.getColumnKey(), cell.getValue());
      }
      return this;

  }

  static class MeanAndVariance {
    private final double mean;
    private final double variance;

    MeanAndVariance(double mean, double variance) {
      this.mean = mean;
      this.variance = variance;
    }

    @Override
    public int hashCode() {
      return Doubles.hashCode(mean) * 31 + Doubles.hashCode(variance);
    }
  }

  enum VarianceAlgorithm {
    DO_NOT_COMPUTE {
      @Override

      return null; // Wrong number of parts
    }

    byte[] bytes = new byte[IPV4_PART_COUNT];
    int start = 0;
    // Iterate through the parts of the ip string.
    // Invariant: start is always the beginning of an octet.
    for (int i = 0; i < IPV4_PART_COUNT; i++) {
      int end = ipString.indexOf(IPV4_DELIMITER, start);
      if (end == -1) {
        end = ipString.length();
      }

  /** Resolver for resolving parameter and field types with {@link #runtimeType} as context. */
  @LazyInit @CheckForNull private transient TypeResolver invariantTypeResolver;

  /** Resolver for resolving covariant types with {@link #runtimeType} as context. */
  @LazyInit @CheckForNull private transient TypeResolver covariantTypeResolver;

  /**
   * Constructs a new type token of {@code T}.
   *