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Here are some examples where I had to use cast or notNull in my project.

Which problems are not solved yet in 0.8? -- ArjanB

   java.util.List<Handle> handles = new ArrayList();
      ConnectionHandle ch;
      ch = cast(handles.get(handles.size()-1));
ConnectionHandle extends Handle

How do you know that the last element has the right type? -- DanielBonniot

The draggable points of a Connection are Handles. The first and the last are connected to flowlets, thats why they are called ConnectionHandles, and the handles in between are EllbowHandles. There can be zero or more EllbowHandles?. I defined it like this. -- AlexGreif

Interesting. Here it is the logic of the system that guarantees the cast will not fail. I think you could take into account this logic in the types. One possibility is to keep a reference to the handles on both extremities in fields of type ConnectionHandle. If you wish, you can encapsulate the handles in a Handle class:

class Handles
  List<Handle> handles;
  ConnectionHandle first;
  CollectionHandle last;
// Some methods to manipulate handles.

Interesting too. But IMO not very nice design, because the class Figure (the super class of all) has the member List<Handle> handles. And I want to store all handles in one List. A Connection has this speciality that it has two types of handles. Besides this I can iterate easily if all Handles are in one List. -- AlexGreif

OK. Then could you add in Connection the two fields, while keeping everything in the list? -- Daniel

Then the info is redundant, and I have to update both. Where do I profit if I introduce two fields? -- Alex

The type information is not redundant. A benefit is that you make sure that there is no cast, that might fail at runtime in some strange condition. On the other hand, it is true that you then need to guarantee that the value in the field is the same as in the list. If you have a set method in the parent to build the list, then you could override it to set the fields when the index is 0 or length - 1, and the call super.

I agree that the benefit is discutable. An alternative is to use the cast. In a future version of Nice, I could imagine that you could do:

Hand h = cast(handles.get(handles.size()-1));
assert h instanceof ConnectionHandle;
// Now you can use h as a ConnectionHandle.

      ?Figure figure = getFrontFlowWindowDrawArea().getDrawing().getFigureExcept(
                  mousePos, this);
      if (figure == null)   return;
      Flowlet flowlet = cast(figure);
Flowlet extends Figure

The problem is now like with ConnectionHandle?. Could you modify the design a bit, so that the type of the expression is Flowlet and not Figure?

No. Figure is the superclass of all drawable Items. Flowlets are items that can be connected together. Connections cannot be connected. Thats why they are Figures.

Here is the class hierarchy:
Figure <- PolygonFigure
PolygonFigure <- Flowlet
PolygonFigure <- Connection
-- AlexGreif

          if (connectorListeners == null)   return;
      for (Iterator<ConnectorListener> iter = notNull(connectorListeners).iterator(); iter.hasNext();) {
         ConnectorListener ltnr =;
         if (figure.getHandles().contains(cast(ltnr))) {
            //println(" deleted: " + ltnr);
Isn't this code equivalent to the following -- ArjanB

I think Arjan is right, I will test it -- AlexGreif

          if (connectorListeners == null)   return;

You need notNull, because connectorListeners is not a local variable. This is explained in OptionTypes. A solution is there too.

What is the type of figure.getHandles() ?

   java.util.List<Handle> getHandles() = handles;

ConnectorHandle, that extends Handle implements ConnectorListener

Interesting. So you have a value of type A, a List<B> with B extends A, and you want to know if the value is in the list, right? We were discussing with Arjan if this is ever needed. The point is, it can only be true if the value is of type B. But now I can believe this is really useful.

If it is really this, we will change the type of contains in nice.lang. After that, you won't need this cast. -- DanielBonniot

I changed the retypings so the cast isn't needed anymore in the next DevelopmentVersion -- Arjan

Another example to show why contains should be contravariant.

<T,U,V | U <: T, V <: T> Set<T> intersection(Set<U>, Set<V>);

intersection<T,U,V>(s1@Set, s2@Set) {
   Set<T> res = new HashSet();
   if (s1.size() < s2.size()) {
       for(U elem : s1) 
           if (s2.contains(elem)) res.add(elem);
   } else {
       for(V elem : s2) 
           if (s1.contains(elem)) res.add(elem);
   return res;
Should I reapply that patch then Daniel? -- ArjanB

OK! Doesn't removeAll also need to get a contravariant type? -- DanielBonniot

Yes but if I'm consistent then should almost all methods of the collection be retyped to get a co/contra-variant type. the methods in Map will get uglier retyping then.

<K, K0, V0, V | K <: K0, V <: ?V0> ?V0 get(java.util.Map<K, V>, K0) =
  native Object java.util.Map.get(Object);

Yes, it makes sense. JSR 14 (Java Generics) does use Object for the key in get. So we are still more strict and safe than them.

I don't think it matters too much if the retypings become more complicated, especially if it means they will be usable in more situations. -- Daniel

Ok but I'm not sure about one kind of retypings:

<K, K0, V | K <: K0> java.util.Set<K0> keySet(java.util.Map<K, V>) =
  native java.util.Set java.util.Map.keySet();

this is safe because a keyset is read/remove only. -- Arjan

Are you not sure it is safe, or if it is useful? I checked the javadoc, it is true that you cannot add elements, so this must be safe. I'm not sure how useful it will be, but it does not harm to do it. Maybe it will turn out useful in some situations. And we can say that Nice allows this, while Java 1.5 cannot! :-) -- Daniel

It reminds me that I still should write something about retyping, at least the is no shortage of examples -- ArjanB

   getFromConnection(state) {
      java.util.List<Connection> followings = this.getFromConnections();
      for (int i = 0; i < followings.size(); i++) {
         DecisionConnection connection = cast(followings.get(i));
         if (connection.getState() == state) {
            return connection;
      return null;
DecisionConnection extends Connection

Why don't you declare that this.getFromConnections() returns a List<DecisionConnection>? -- DanielBonniot

The code above is in DecisionFlowlet, but this.getFromConnections() is declared in Flowlet that returns Connection objects.

Is getFromConnections reimplemented for DecisionFlowlet? -- DanielBonniot

No. Only implemented in Flowlet class. Thus I have to return a List with the lowest common denom.. --Main.AlexGreif

Should I use :

      java.util.List<DecisionConnection> followings = cast(this.getFromConnections());
-- AlexGreif

It's possible, but that's still a cast.

Note that using get on a List in a loop can be bad, in case the List is a linked list: you will traverse n**2 elements, instead of n. This is ideal for the new for syntax of version 0.7.8:

   getFromConnection(state) {
      java.util.List<DecisionConnection> followings = cast(this.getFromConnections());
      for (DecisionConnection connection : followings) {
         if (connection.getState() == state) {
            return connection;
      return null;
-- DanielBonniot

OOps did I miss that? Is it documented somewhere? -- AlexGreif

I think it's in the changelog only, at the moment. You see, Arjan, documentation is needed! :-) -- DanielBonniot

It should be in the manual that you need to look in the changelog. ;-) -- ArjanB

:-) Seriously, Arjan, you should write something to put in the Statements section. -- Daniel

more to come...

-- AlexGreif - 22 Apr 2003

OK, thanks. I'll treat them progressively.

For those that are solved, if you feel that some documentation should be added (for the numeric types, I updated the User manual, so that it compares with the Java syntax). After that they can be deleted.

Some will probably need to stay, when they are a good example of a typing problem, and how to solve it. So we can keep those there.

-- DanielBonniot - 22 Apr 2003

Topic NiceCasts . { Edit | Attach | Ref-By | Printable | Diffs | r1.24 | > | r1.23 | > | r1.22 | More }
Revision r1.24 - 06 Jun 2003 - 20:26 GMT - AlexGreif
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