Flows and Edges

UML 2 uses the terms flow and edge synonymously to describe the connections between two actions. The simplest kind of edge is the simple arrow between two actions. You can give an edge a name if you like, but most of the time, a simple arrow will suffice.

If you're having difficulty routing lines, you can use connectors, which simply save you having to draw a line the whole distance. When you use connectors, you must use them in pairs: one with incoming flow, one with an outgoing flow, and both with the same label. I tend to avoid using connectors if at all possible, as they break up the visualization of the flow of control.

The simplest edges pass a token that has no meaning other than to control the flow. However, you can also pass objects along edges; the objects then play the role of tokens, as well as carry data. If you are passing an object along the edge, you can show that by putting a class box on the edge, or you can use pins on the actions, although pins imply some more subtleties that I'll describe shortly.

All the styles shown in Figure 11.7 are equivalent; you should use whichever conveys best what you are trying to communicate. Most of the time, the simple arrow is quite enough.