16.2. Responsibilities and Interaction Diagrams

The purpose of this chapter is to help methodically apply fundamental principles for assigning responsibilities to objects. This will often be done while programming. Within the UML artifacts, a common context where these responsibilities (implemented as methods) are considered is during the creation of interaction diagrams (which are part of the UP Design Model), whose basic notation we examined in the previous chapter.

Figure 16.1 indicates that Sale objects have been given a responsibility to create Payments, which is invoked with a makePayment message and handled with a corresponding makePayment method. Furthermore, the fulfillment of this responsibility requires collaboration to create the SalesLineItem object and invoke its constructor.

In summary, interaction diagrams show choices in assigning responsibilities to objects. When created, decisions in responsibility assignment are made, which are reflected in what messages are sent to different classes of objects. This chapter emphasizes fundamental principles—expressed in the GRASP patterns—to guide choices in where to assign responsibilities. These choices are reflected in interaction diagrams.