UML Is Not Enough

Although the UML provides quite a considerable body of various diagrams that help to define an application, it's by no means a complete list of all the useful diagrams that you might want to use. In many places, different diagrams can be useful, and you shouldn't hesitate to use a non-UML diagram if no UML diagram suits your purpose.

Figure 1.3, a screen flow diagram, shows the various screens on a user interface and how you move between them. I've seen and used these screen flow diagrams for many years. I've never seen more than a very rough definition of what they mean; there isn't anything like it in the UML, yet I've found it a very useful diagram.

Table 1.2 shows another favorite: the decision table. Decision tables are a good way to show complicated logical conditions. You can do this with an activity diagram, but once you get beyond simple cases, the table is both more compact and more clear. Again, many forms of decision tables are out there. Table 1.2 divides the table into two sections: conditions above the double line and consequences below it. Each column shows how a particular combination of conditions leads to a particular set of consequences.

Table 1.2. A Decision Table

Premium customer	X	X	Y	Y	N	N
Priority order	Y	N	Y	N	Y	N
International order	Y	Y	N	N	N	N
Fee	$150	$100	$70	$50	$80	$60
Alert rep	•	•	•

You'll run into various kinds of these things in various books. Don't hesitate to try out techniques that seem appropriate for your project. If they work well, use them. If not, discard them. (This is, of course, the same advice as for UML diagrams.)