}

    @Override
    public boolean containsKey(@Nullable Object key) {
      return backingSet().contains(key);
    }

    @Override
    public @Nullable V get(@Nullable Object key) {
      return getOrDefault(key, null);
    }

    @Override
    public @Nullable V getOrDefault(@Nullable Object key, @Nullable V defaultValue) {
      if (Collections2.safeContains(backingSet(), key)) {

  }

  private static <K extends @Nullable Object, V extends @Nullable Object>
      @Nullable Entry<K, V> unmodifiableOrNull(@Nullable Entry<K, ? extends V> entry) {
    return (entry == null) ? null : unmodifiableEntry(entry);
  }

  @GwtIncompatible // NavigableMap
  private static final class UnmodifiableNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>

    int nextSegmentIndex;
    int nextTableIndex;
    @Nullable Segment<K, V> currentSegment;
    @Nullable AtomicReferenceArray<ReferenceEntry<K, V>> currentTable;
    @Nullable ReferenceEntry<K, V> nextEntry;
    @Nullable WriteThroughEntry nextExternal;
    @Nullable WriteThroughEntry lastReturned;

    HashIterator() {
      nextSegmentIndex = segments.length - 1;

      K key, int hash, V value, @Nullable ReferenceEntry<K, V> next) {
    DummyEntry<K, V> entry = DummyEntry.create(key, hash, next);
    DummyValueReference<K, V> valueRef = DummyValueReference.create(value);
    entry.setValueReference(valueRef);
    return entry;
  }

  static class DummyEntry<K, V> implements ReferenceEntry<K, V> {
    private @Nullable K key;
    private final int hash;

      K key, int hash, V value, @Nullable ReferenceEntry<K, V> next) {
    DummyEntry<K, V> entry = DummyEntry.create(key, hash, next);
    DummyValueReference<K, V> valueRef = DummyValueReference.create(value);
    entry.setValueReference(valueRef);
    return entry;
  }

  static class DummyEntry<K, V> implements ReferenceEntry<K, V> {
    private @Nullable K key;
    private final int hash;