API fields previously capable of storing and returning either `[]` and `null` via JSON API requests (for example, the Endpoints `subsets` field)
can now store only `null` when created using the protobuf content-type or stored in etcd using protobuf serialization (the default in 1.6).

  @GwtIncompatible
  @J2ktIncompatible
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    Comparator<Object> valueComparator = (Comparator<Object>) stream.readObject();
    int keyCount = stream.readInt();
    if (keyCount < 0) {

      long normalizedNum = num >> tz;
      shift += tz;
      // Adjust floor(log2(num)) + 1.
      int normalizedBits = bits - tz;
      // If it won't fit in a long, then we store off the intermediate product.
      if (normalizedBits + productBits >= Long.SIZE) {
        bignums.add(BigInteger.valueOf(product));
        product = 1;
        productBits = 0;
      }

  @GwtIncompatible
  @J2ktIncompatible
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    Comparator<Object> valueComparator = (Comparator<Object>) stream.readObject();
    int keyCount = stream.readInt();
    if (keyCount < 0) {

      default:
        /*
         * The current implementation will end up using entryArray directly, though it will write
         * over the (arbitrary, potentially mutable) Entry objects actually stored in entryArray.
         */
        return RegularImmutableBiMap.fromEntries(entryArray);
    }
  }

  ImmutableBiMap() {}

  /**
   * {@inheritDoc}
   *

   */

  private transient Map<K, Collection<V>> map;
  private transient int totalSize;

  /**
   * Creates a new multimap that uses the provided map.
   *
   * @param map place to store the mapping from each key to its corresponding values
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  protected AbstractMapBasedMultimap(Map<K, Collection<V>> map) {
    checkArgument(map.isEmpty());

      while (elements.hasNext()) {
        add(elements.next());
      }
      return this;
    }

    @CanIgnoreReturnValue
    @SuppressWarnings("unchecked") // ArrayBasedBuilder stores its elements as Object.
    Builder<E> combine(Builder<E> other) {
      if (hashTable != null) {
        for (int i = 0; i < other.size; ++i) {

    // Unfortunately this is pasted from ImmutableCollection.Builder.
    private static int expandedCapacity(int oldCapacity, int minCapacity) {
      if (minCapacity < 0) {
        throw new AssertionError("cannot store more than MAX_VALUE elements");
      }
      // careful of overflow!
      int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
      if (newCapacity < minCapacity) {

    // Unfortunately this is pasted from ImmutableCollection.Builder.
    private static int expandedCapacity(int oldCapacity, int minCapacity) {
      if (minCapacity < 0) {
        throw new AssertionError("cannot store more than MAX_VALUE elements");
      }
      // careful of overflow!
      int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
      if (newCapacity < minCapacity) {

      this.dedupedElements = Arrays.copyOf(toCopy.dedupedElements, toCopy.dedupedElements.length);
      this.distinct = toCopy.distinct;
    }

    /**
     * Resizes internal data structures if necessary to store the specified number of distinct
     * elements.
     */
    private void ensureCapacity(int minCapacity) {
      if (minCapacity > dedupedElements.length) {
        int newCapacity =