Design Patterns Explained

Reading a Sequence Diagram.
As I discussed in Chapter 2, “The UML—The Unified Modeling
Language,” the diagram in Figure 9-4 is a special kind of interaction
diagram called a Sequence Diagram. It is a common diagram in
the UML. Its purpose is to show the interaction of objects in the
system.
Each box at the top represents an object. It may be named or
not.
If an object has a name, it is given to the left of the colon.
The class to which the object belongs is shown to the right of
the colon. Thus, the middle object is named myRectangle
and is an instance of V1Rectangle.
You read the diagram from the top down. Each numbered statement
is a message sent from one object to either itself or to
another object.
The sequence starts out with the unnamed Client object
calling the draw method of myRectangle.
This method calls its own drawLine method four times
(shown in steps 2, 4, 6, and 8). Note the arrow pointing back
to the myRectangle in the timeline.
drawLine calls DP1’s draw_a_line. This is shown in steps 3,
5, 7 and 9.
Even though the Class Diagram makes it look like there are many
objects, in reality, I am only dealing with three objects (see Figure
9-5):
The client using the rectangle
The V1Rectangle object
The DP1 drawing program
Chapter 9 The Bridge Pattern 131