@Override
    public <E> E[] toArray(E[] a) {
      return toArrayList(this).toArray(a);
    }
  }

  private static <E> ArrayList<E> toArrayList(Collection<E> c) {
    // Avoid calling ArrayList(Collection), which may call back into toArray.
    ArrayList<E> result = new ArrayList<>(c.size());
    Iterators.addAll(result, c.iterator());
    return result;
  }

    @Override
    public <E> E[] toArray(E[] a) {
      return toArrayList(this).toArray(a);
    }
  }

  private static <E> ArrayList<E> toArrayList(Collection<E> c) {
    // Avoid calling ArrayList(Collection), which may call back into toArray.
    ArrayList<E> result = new ArrayList<>(c.size());
    Iterators.addAll(result, c.iterator());
    return result;
  }

  final class KeySet extends AbstractCacheSet<K> {