android/guava/src/com/google/common/collect/CompactHashSet.java
private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeInt(size);
    for (int i = firstEntryIndex(); i >= 0; i = getSuccessor(i)) {
      stream.writeObject(elements[i]);
    }
  }
private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeInt(size);
    for (int i = firstEntryIndex(); i >= 0; i = getSuccessor(i)) {
      stream.writeObject(elements[i]);
    }
  }
The serial form currently mimics Android's java.util.HashSet version, e.g. see
http://omapzoom.org/?p=platform/libcore.git;a=blob;f=luni/src/main/java/java/util/HashSet.java