Summary

In this chapter, we showed how requirements do not align with objects, leading to difficulties in the object-oriented way of developing software. Requirements do not align with objects because the units of interest in the requirements for a software system are not the same as the units of interest in an object-oriented model of the same system. To get from one to the other requires a transformation of some kind that hurts evolution, comprehensibility, reuse and traceability. We walked through applying good object-oriented design practices in designing a simple expression-evaluation system. We found that problems arose, both in the initial design and upon evolution, because, ultimately, we had to adhere to objects as the main unit of modularity. In the next chapter, we apply the aspect-oriented Theme approach for designing the same system. We will see that by using themes, we can overcome the difficulties that are present with straight object orientation.