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In our use case, start car usage , this also applies
to all the steps up to and including the step unlock car ( Figure 2.45 ). However,
the subsequent steps cannot be arranged in a fi xed sequence. A customer can
fi rst remove the key and then enter the PIN, or vice-versa. The two steps are
independent of one another. They can occur in an arbitrary sequence, perhaps even
concurrently in case a customer is willing and able to go into contortions
( Figure 2.55 ).
In activity diagrams we express independent fl ows by means of a fork node,
which is denoted by a black bar at which a fl ow ends, and from which an arbitrary
number of independent fl ows start. In other words: the fl ow is ??? forked ???
( Figure 2.56 ).
As soon as the independence is no longer desired, we have to synchronize the
fl ows. The join node is also denoted by a black bar. In contrast to the fork notation,
however, it has several incoming fl ows and only one single outgoing fl ow ( Figure
2.56 ). The outgoing fl ow doesn ??™ t start before all incoming fl ows have arrived.


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