The Object Teams Blog

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Object Teams rocks :)

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This is a story of code evolution at its best. About re-use and starting over.

During the last week or so I modernized a part of the Object Teams Development Tooling (OTDT) that had been developed some 5 years ago: the type hierarchy for OT/J. I’ll mention the basic requirements for this engine in a minute. While most of the OTDT succeeds in reusing functionality from the JDT, the type hierarchy was implemented as a full replacement of the original. This is a pretty involved little machine, which took weeks and months to get right. It provides its logic to components like Refactoring and the Type Hierarchy View.

On the one hand this engine worked well for most uses, but over so many years we did not succeed to solve two remaining issues:

Give a faithful implementation for getSuperclass()
This is tricky because a role class in OT/J can have more than one superclass. Failing to implement this method we could not support the “traditional” mode of the hierarchy view that shows both the tree of subclasses of a focus type plus the path of superclasses up to Object (this upwards path relies on getSuperclass).
Support region based hierarchies
Here the type hierarchy is not only computed for supertypes and subtypes of one given focus type, but full inheritance structure is computed for a set of types (a “region”). This strategy is used by many JDT Refactorings, and thus we could not precisely adapt some of these for OT/J.

In analyzing this situation I had to weigh these issues:

  • In its current state the implementation strategy was a show stopper for one mode of the type hierarchy view and for precise analysis in several refactorings.
  • Adding a region based variant of our hierarchy implementation would mean to re-invent lots of stuff, both from the JDT and from our own development.
  • All this seemed to suggest to discard our own implementation and start over from scratch.
Start over I did, but not from scratch but from the wealth of a working JDT implementation.

Object Teams to the rescue: Let’s re-build Rome in ten days.

As mentioned in my previous post, the strength of Object Teams lies in building layers: each module sits in one layer, and integration between layers is given by declarative bindings:

Applying this to the issue at hand we now actually have three layers with quite different structures:

Java Model

The bottom layer is the Java model that implements the containment tree of Jave elements: A project contains source folders, containing packages, containing compilation units, containing types containing members. In this model each Java type is represented by an instance of IType

Java Type Hierarchy

This engine from the JDT maintains the graph of inheritance information as a second way for navigating between ITypes. Interestingly, this module pretty closely simulates what Object Teams does natively, I may come back to that in a later post.

Object Teams Type Hierarchy

As an extension of Java, OT/J naturally supports the normal inheritance using extends, but there is a second way how an inheritance link can be established: based on inheritance of the enclosing team:

team class EcoSystem {
   protected class Project { }
   protected class IDEProject extends Project { }
team class Eclipse extends EcoSystem {
   protected class Project { }
   protected class IDEProject extends Project { }

Here, Eclipse.Project is an implicit subclass of EcoSystem.Project simply because Eclipse is a subclass of EcoSystem and both classes have the same simple name Project. I will not go into motivation and consequences of this language design (that’ll be a separate post — which I actually promised many weeks ago).

Looking at the technical challenge we see that the implicit inheritance in OT/J adds a third layer, in which classes are connected in yet another graph.

Three Layers — Three Graphs

Looking at the IType representation of Eclipse.IDEProject we can ask three questions:

Question Code Answer
What is your containing element? type.getParent() Eclipse
What is your superclass? hierarchy.getSuperclass(type) Eclipse.Project
What is your implicit superclass? ?? EcoSystem.Project

Each question is implemented in a different layer of the system. Things get a little complicated when asking a type for all its super types, which requires to collect the answers from both the JDT hierarchy layer and the OT hierarchy. Yet, the most tricky part was giving an implementation for getSuperclass().

An "Impossible" Requirement

There is a hidden assumption behind method getSuperclass() which is pervasive in large parts of the implementation, especially most refactorings:

When searching all methods that a type inherits from other types, looping over getSuperclass() until you reach Object will bring you to all the classes you need to consider, like so:

IType currentType = /* some init */;
while (currentType != null) {
   findMethods(currentType, /*some more arguments*/);
   currentType = hierarchy.getSuperclass(currentType);

There are lots and lots of places implemented using this pattern. But, how do you do that if a class has multiple superclasses?? I cannot change all the existing code to use recursive functions rather than this single loop!

Looking at Eclipse.IDEProject we have two direct superclasses: Eclipse.Project (normal inheritance, “extends”) and EcoSystem.IDEProject (OT/J implicit inheritance), which cannot both be answered by a single call to getSuperclass(). The programming language theory behind OT/J, however, has a simple answer: linearization. Thus, the superclasses of Eclipse.IDEProject are:

  • Eclipse.IDEProject → EcoSystem.IDEProject → Eclipse.Project → EcoSystem.Project

… in this order. And this is how this shall be rendered in the hierarchy view:

The final callenge: what should this query answer:


According to the above linearization we should answer: Eclipse.Project, but only if we are in the context of the superclass chain of Eclipse.IDEProject. Talking directly to EcoSystem.IDEProject we should get EcoSystem.Project! In other words: the function needs to be smarter than what it can derive from its arguments.

Layer Instances for each Situation

Let’s go back to the layer thing:

At the bottom you see the Java model (as rendered by the package explorer). In the top layer you see the OT/J type hierarchy (lets forget about the middle layer for now). Two essential concepts can be illustrated by this picture:

  • Each layer is populated with objects and while each layer owns its objects, those objects connected with a red line between layers are almost the same, they represent the same concept.
  • The top layer can be instantiated multiple times: for each focus type you create a new OT/J hierarchy instance, populated with a fresh set of objects.

It is the second bullet that resolves the “impossible” requirement: the objects within each layer instance are wired differently, implementing different traversals. Depending on the focus type, each layer may answer the getSuperclass(type) question differently, even for the same argument.

The first bullet answers how these layers are integrated into a system: Conceptually we are speaking about the same Java model elements (IType), but we superimpose different graph structure depending on our current context.

All layers basically talk about the same objects,
but in each layer these objects are connected in a specific way as suites for the task at hand.

Inside the hierarchy layer, we actually do not handle IType instances directly, but we have roles that represent one given IType each. Those roles contain all the inheritance links needed for answering the various questions about inheritance relations (direct/indirect, explicit/implicit, super/sub).

A cool thing about Object Teams is, that having different sets of objects in different layers (Team teams) doesn’t make the program more complex, because I can pass an object from one layer into methods of another layer and the language will quite automagically translate into the object that sits at the other end of that red line in the picture above. Although each layer has its own view, they “know” that they are basically talking about the same stuff (sounds like real life, doesn’t it?).

Summing up

OK, I haven’t shown any code of the new hierarchy implementation (yet), but here’s a sketch of before-vs.-after:

Code Size
The new implementation of the hierarchy engine has about half the size of the previous implementation (because it need not repeat anything that’s already implemented in the Java hierarchy).
The previous implementation had to be individually integrated into each client module that normally uses Java hierarchies and then should use an OT hierarchy instead. After the re-implementation, the OT hierarchy is transparently integrated such that no clients need to be adapted (accounting for even more code that could be discarded).
Using the new implementation, getSuperclass() answers the correct, context sensitive linearization, as shown in the screenshot above, which the old implementation failed to solve.
Region based hierarchies
The old implementation was incompatible with building a hierarchy for a region. For the new implementation it doesn’t matter whether it’s built for a single focus type or for a region, so, many clients now work better without any additional efforts.

The previous implementation only scratched at the surface – literally worked around the actual issue (which is: the Java type hierarchy is not aware of OT/J implicit inheritance). The new solution solves the issue right at its core: the new team OTTypeHierarchies assists the original type hierarchy (such that its answers indeed respect OT/J’s implicit inheritance). By performing this adaptation at the issue’s core, the solution automatically radiates to all clients. So I expect that investing a few days in re-writing the implementation will pay off in no time. Especially, improving the (already strong) refactoring support for OT/J is now much, much easier.

Lessons learned: when your understanding of a problem improves, you’ll be able to discard your old workarounds and move the solution closer to the core. This reduces code size, makes the solution more consistent, enables you to solve issues you previously weren’t able to solve, and transparently provides the solution to a wider range of client modules.
Moving your solution into the core could easily result in a design were a few bloated and tangled core modules do all the work, mocking the very idea of modularity. This can be avoided by a technology that is based on some concept of perspectives and self-contained layers, as supported by teams in OT/J.

Need I say, how much fun this re-write was? 🙂


Written by Stephan Herrmann

August 18, 2010 at 22:52

2 Responses

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  1. I don’t know if you know Scala, but linearization is exactly what this language uses to compute the type hierarchy of classes that extends many traits. Your solution is definitively the right one ! 🙂

    Nicolas Lalevée

    August 20, 2010 at 17:30

  2. @Nicolas: Sure do I know Scala, and sure did I not invent class linearization. I can’t remember if I talked about this with Martin Odersky. I learned many cool things about inheritance from Erik Ernst, and I know Erik and Martin are very well informed about each other’s contributions. So it’s a community of language designers whose ideas actually converge in some topics, which is indeed a nice place to be at.

    Next question might be: how are inheritance hierarchies visualized in the Scala IDE? I haven’t checked.

    Next question might be: how did they implement their hierarchy view (if they have one), but that wouldn’t be an even comparison, because the Scala IDE is green field development, whereas the Object Teams Development Tooling started from the full blown JDT 🙂


    August 20, 2010 at 20:12

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