@Parameters
  public static Collection<Object[]> parameters() {
    Comparator<String> nullsLast = Ordering.natural().nullsLast();

    return Arrays.asList(
        new Object[][] {
          {new MapIteratorCache<String, String>(new HashMap<String, String>())},
          {new MapIteratorCache<String, String>(new TreeMap<String, String>(nullsLast))},
          {new MapRetrievalCache<String, String>(new HashMap<String, String>())},

  @Parameters
  public static Collection<Object[]> parameters() {
    Comparator<String> nullsLast = Ordering.natural().nullsLast();

    return Arrays.asList(
        new Object[][] {
          {new MapIteratorCache<String, String>(new HashMap<String, String>())},
          {new MapIteratorCache<String, String>(new TreeMap<String, String>(nullsLast))},
          {new MapRetrievalCache<String, String>(new HashMap<String, String>())},

  @Override
  @SuppressWarnings("unchecked") // TODO(kevinb): the right way to explain this??
  public <S extends Comparable<?>> Ordering<@Nullable S> nullsLast() {
    Ordering<@Nullable Comparable<?>> result = nullsLast;
    if (result == null) {
      result = nullsLast = super.<Comparable<?>>nullsLast();
    }
    return (Ordering<@Nullable S>) result;
  }

  @Override

    return ordering.<T>reverse().<@NonNull S>nullsLast();
  }

  @SuppressWarnings("unchecked") // still need the right way to explain this
  @Override
  public <S extends @Nullable T> Ordering<@Nullable S> nullsFirst() {
    return (Ordering<@Nullable S>) this;
  }

  @Override
  public <S extends @Nullable T> Ordering<@Nullable S> nullsLast() {
    return ordering.<@NonNull S>nullsLast();
  }

  @Override

    return ordering.<@NonNull S>nullsFirst();
  }

  @SuppressWarnings("unchecked") // still need the right way to explain this
  @Override
  public <S extends @Nullable T> Ordering<@Nullable S> nullsLast() {
    return (Ordering<@Nullable S>) this;
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    }

    return ordering.<@NonNull S>nullsFirst();
  }

  @SuppressWarnings("unchecked") // still need the right way to explain this
  @Override
  public <S extends @Nullable T> Ordering<@Nullable S> nullsLast() {
    return (Ordering<@Nullable S>) this;
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    }

        .thenComparing(v -> v.minor)
        .thenComparing(v -> v.revision)
        .thenComparing((QualifiedVersion v) -> v.qualifier, Comparator.nullsLast(Comparator.naturalOrder()));

    @Override
    public int compareTo(QualifiedVersion other) {
        return COMPARATOR.compare(this, other);
    }

    public boolean isBefore(QualifiedVersion other) {