UML 2.0 in a Nutshell

Combined Fragments

Often there are times when a particular sequence of event occurrences has special constraints or properties. For example, you may have a critical region within your interaction where a set of method calls must execute atomically, or a loop that iterates over a collection. UML calls these smaller pieces interaction fragments.

Interaction fragments by themselves aren't terribly interesting, however UML allows you to place them in a container called a combined fragment (it's called a combined fragment even if you have only one interaction fragment in there). Once they are placed in such a container, UML allows you to specify additional detail for each fragment, or how several fragments relate to each other.

Each combined fragment is made up of an interaction operator and one or more interaction fragments, which are the interaction operands. An interaction operator specifies how the interaction operands should be interpreted. The various interaction operators are described in detail later in this chapter.

As you do with full interactions, you show a combined fragment as a rectangle, with the interaction operator in a pentagon in the upper left and the interaction operands inside the rectangle. Figure 10-17 shows a combined fragment representing a critical section of code. The code must be executed atomically because of the interaction operand critical. This and other interaction operators are described in "Interaction Operators."

Figure 10-17. An example combined fragment ...