* Object)} are even more convenient.
   *
   * <p>Builder instances can be reused - it is safe to call {@link #buildOrThrow} multiple times to
   * build multiple tables in series. Each table is a superset of the tables created before it.
   *
   * @since 11.0
   */
  @DoNotMock
  public static final class Builder<R, C, V> {

  }

  /**
   * Returns a synchronized (thread-safe) multimap backed by the specified multimap. In order to
   * guarantee serial access, it is critical that <b>all</b> access to the backing multimap is
   * accomplished through the returned multimap.
   *
   * <p>It is imperative that the user manually synchronize on the returned multimap when accessing
   * any of its collection views:
   *
   * <pre>{@code

    return new HashMap<>(map);
  }

  /**
   * Creates a {@code HashMap} instance, with a high enough "initial capacity" that it <i>should</i>
   * hold {@code expectedSize} elements without growth. This behavior cannot be broadly guaranteed,
   * but it is observed to be true for OpenJDK 1.7. It also can't be guaranteed that the method
   * isn't inadvertently <i>oversizing</i> the returned map.
   *

   * @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;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

  // We keep it to minimize diffs.
  @SuppressWarnings("unchecked")
  public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
    /*
     * TODO(lowasser): consider checking for ImmutableAsList here
     * TODO(lowasser): consider checking for Multiset here
     */
    // Don't refer to ImmutableSortedSet by name so it won't pull in all that code

   *
   * <p>Elements appear in the resulting list in the same order they were added to the builder.
   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple lists in series. Each new list contains all the elements of the ones created before
   * it.
   *
   * @since 2.0
   */
  public static final class Builder<E> extends ImmutableCollection.ArrayBasedBuilder<E> {
    /**

   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple multisets in series.
   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.Builder<E> {
    /*
     * `contents` is null only for instances of the subclass, ImmutableSortedMultiset.Builder. That
     * subclass overrides all the methods that access it here. Thus, all the methods here can safely

   *         .put("one", 1)
   *         .putAll("several", 1, 2, 3)
   *         .putAll("many", 1, 2, 3, 4, 5)
   *         .build();
   * }</pre>
   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple multimaps in series. Each multimap contains the key-value mappings in the previously
   * created multimaps.
   *
   * @since 2.0
   */

            .put("three", 3)
            .put("one", 1)
            .put("five", 5)
            .put("four", 3)
            .put("four", 5)
            .put("four", 4) // this should win because it's last
            .put("two", 2);
    assertMapEquals(
        builder.buildKeepingLast(), "one", 1, "two", 2, "three", 3, "four", 4, "five", 5);
    try {
      builder.buildOrThrow();

        }
        /*
         * requireNonNull is safe because our callers always pass non-null arguments. Each element
         * of the array becomes null only when we iterate past it and then clear it.
         */
        I result = requireNonNull(elements[index]);
        elements[index] = null;
        index++;
        return result;
      }
    };
  }

  /**