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Brief notes: Borg Calendar case study exercise class

We based this class on the Borg Calendar open source Java program: I encourage you to browse its code on Sourceforge and think about aspects of its design beyond those we discussed. Its design is not beyond criticism, but it is fairly clean and easy to understand.

The full version of the software requirements specification we used can be found here and the slides I used showing the exercises you did (although we didn't get to the third) are here.

A few of the main points discussed were:

• the relationship between Task and Subtask. It's an association; more specifically, an aggregation; yet more specifically, a composition.

  Why not a generalization (inheritance)? Because there isn't a clear Is-A relationship between them. The one that is tempting (and might be appropriate in other applications) is the idea that maybe "a Subtask is a Task". However, we know from the SRS that a Subtask does not have states other than open and closed -- for example, maybe you can defer a Task but not a Subtask. This suggests that it probably isn't true that any context that would work with a Task would also work with a Subtask, which is what we'd need for a generalization relationship that way round. Technically, we might have more luck with "a Task is a Subtask" but that will probably fail by a Subtask having the capability to contact its parent, which won't (always) make sense for a Task, and besides, it sounds weird, i.e., it doesn't correctly model the concepts in the domain.

• which things that will eventually be implemented as fields in a Java class are represented as attributes in a UML class diagram, and which are represented as associations. The rule: it depends on the type. If the property has a base type (e.g. String) or something that for your purposes is like a base type, e.g. a class you get from a class library rather than having to design it yourself, then represent it as an attribute. If the property has as its type a class that itself is part of your design, then don't represent it as an attribute: show it as an association.
• how to use UML2 sequence diagram fragments loop, alt and opt. Many people seemed unsure about this: you may need to revisit the sequence diagram reading and video page, and/or the revision of the lecture from Inf2C-SE.

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