// no null keys or null values

sclass BijectiveMap<A, B> extends CompactAbstractMap<A, B> {

  new Map<A, B> forward;

  new Map<B, A> backward;

  *() {}

  *(bool ordered) {

    if (ordered) {

      forward = new LinkedHashMap;

      backward = new LinkedHashMap;

    }

  }

  @Override public B put(A a, B b) {

    assertNotNull(a);

    assertNotNull(b);

    A oldKey = backward.get(b);

    if (eq(oldKey, a))

      ret b; // nothing to do

    // remove old mapping of b

    if (oldKey != null)

      forward.remove(oldKey);

    // remove old mapping of a

    B oldValue = forward.get(a);

    if (oldValue != null)

      backward.remove(b);

    // add/overwrite new mapping

    backward.put(b, a);

    ret forward.put(a, b);

  }

  @Override public B remove(O a) {

    B b = forward.get(a);

    if (b != null) {

      forward.remove(a);

      backward.remove(b);

    }

    ret b;

  }

  @Override public B get(O a) {

    ret forward.get(a);

  }

  A inverseGet(B b) {

    ret backward.get(b);

  }

  public Set<A> keySet() { ret immutableSet(forward.keySet()); }

  public Set<B> valueSet() { ret immutableSet(backward.keySet()); }

  public Set<Map.Entry<A, B>> entrySet() { ret immutableSet(forward.entrySet()); }

  public int size() { ret forward.size(); }

  Map<A, B> forwardMap() { ret immutableMap(forward); }

  Map<B, A> backwardMap() { ret immutableMap(backward); }

}