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In Figure 3.11 , we read,
??? Each customer has one or no usage right . ??? Since only one association name
is permitted we can state only one direction in this way, even though the other
direction might be valid. You have to be careful not to confuse the triangle with
the information fl ow.
In the above example, what if the class customer had a usage right , but no
customer should belong to the class usage right ? We would need a uni-directional
association. This property controls the so-called navigation. We denote an arrow at
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the association end to express that it is possible to navigate from one class to the
other. 7
If we continue thinking along these lines in our example, then we should actually
denote an arrow at the other end too, since this direction is also valid. The question
is then why we haven ??™ t used any arrows in Figure 3.11 . The answer is that, by
convention, both navigation directions are possible if no arrows are stated.
You can see a so-called derived association between the usage data class and
the car class in Figure 3.


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