Software Engineering for Students A Programming Approach

24.7 ● Use case driven implementation
24.8 Discussion 319
A use case is a single independent function that a system provides for a user. For example,
the word processor (Appendix A) provides tens of functions to carry out the tasks of
editing, formatting and displaying text. Each of these is a use case. This approach takes
one use case at a time and implements it.
In the word processor example, we might first choose to implement the save file
function. This is a good choice because, once it is implemented, this feature is useful in
providing a mechanism for testing the other use cases. Implementing this function
means single-mindedly concentrating on it to the exclusion of all the other functions.
Only the methods necessary to achieving this end are implemented. Drivers are not
needed and a minimal number of stubs are required.
The use case approach is different to the top-down approach explained above. Topdown
starts with the user interface and implements each of the various functions in a
breadth-first fashion. It creates a forest of trees that grow simultaneously. In contrast, the
use case approach is a depth-first approach. It creates individual trees one after the other.
The stages of the use case approach are:
1. choose an initial use case to implement
2. implement the use case
3. test the use case
4. demonstrate the use case to the client.
These stages are repeated until the user is satisfied with the product.
The strengths of this approach are:
■ the system acts as its own drivers
■ there are immediate and repeated, visible products.
SELF-TEST QUESTION
24.2 Suggest a second use case for implementation.
24.8 ● Discussion
We have assumed in the above explanation that architectural design is complete prior
to incremental implementation. This is one approach, but an alternative is to allow the
architectural structure to emerge from the implementation. This means that the structure
will probably be subject to frequent refactoring (see Chapter 13). This takes some
courage and it might be too risky an approach for a large project.