UML Weekend Crash Course™ - To Parent Directory

152
Saturday Afternoon
Eat your dinner
[Finished your vegetables]
Leave the table
Figure 14-2
A guard condition on a transition
Decisions
The Activity diagram diamond is a decision icon, just as it is in flowcharts. In either diagram,
one arrow exits the diamond for each possible value of the tested condition. The decision
may be as simple as a true/false test (for example, the left-hand illustration in Figure
14-3 asks, “Are there sufficient funds in the customer’s account to cover the withdrawal”).
The decision may involve a choice between a set of options. For example, the
right-hand illustration in Figure 14-3 asks, “Would you like chocolate, vanilla, strawberry, or
rocky road ice cream”
Each option is identified using a guard condition. Each guard condition must be mutually
exclusive so that only one option is possible at any decision point. The guard is placed on
the transition that shows the direction that the logic follows if that condition is true. If
you write code, then you have probably used a case statement to handle this same type of
problem.
[sufficient
funds]
[insufficient
funds]
[chose
chocolate]
[chose
vanilla]
[chose
strawberry]
[chose
rocky road]
Give the
customer the
money
Shake your
finger at the
customer
Serve up
chocolate
ice cream
Serve up
vanilla
ice cream
Serve up
strawberry
ice cream
Serve up
rocky road
ice cream
Figure 14-3 Making a decision
Because every choice at a decision point is modeled with a guard condition, it is possible
to use the conditional logic on transitions leaving an activity as well. For example, in Figure
14-4 the activity of computing the new account balance reveals whether the account is
overdrawn. All the information needed to make the choice is provided by the activity. To
show the choices resulting from an activity, simply model the transitions exiting the activity,
each with a different guard condition.