Parts, Wholes and Sub-Classes

In our view, there are two areas where the object-oriented paradigm is very strong. First, it gives good support for the modelling process, where we use classes and inheritance to build conceptualizations of the real world in a natural way. Second, object-oriented languages give good support for the programming process through the code-sharing made possible by inheritance. Unfortunately, these two uses of inheritance sometimes work against each other, leading to programs that are either difficult to understand, or where a lot of code is replicated. In this paper, we will discuss another important modelling construct- aggregation- that can also be made subject to code-sharing, and this type of code-sharing can often be used without obstructing the modelling.