UML Aggregation with and without arrow head

Question

I always thought that the UML aggregate is defined as a black (filled) diamond at the beginning of a path and no arrow head that the end:

|--------|        |         


        

Answer 1

One arrow means the association is navigable this way. No arrows means the association is navigable BOTH ways. Two arrows are omitted.

It could be a problem, because two ends with undefined navigability look out the same way, but it is the standard.

You can read more thoroughly about associations/navigability/aggregations in this my answer https://stackoverflow.com/a/21478862/715269