在UML class图中，组合可以是双向的吗？

Question

Can composition be bidirectional in a way that both classes are aware of each other?

And if not, what is the default direction of composition?

Answer 1

You should distinguish navigability and aggregation. Arrow and diamond.

Arrow A->B means only that B is reachable from A in some simple way. If A contains a composition of B, it means that

the composite object has responsibility for the existence and storage of the composed objects (parts).

(citation from OMG Unified Modeling Language TM (OMG UML) - p.109)

So, can congregation have bi-directional navigability?

Yes. It is quite normal.

If, for example, you have decided to destroy B in some of its functions, you MUST reach A and destroy it from there. So, composition has bi-directional navigability often enough. Notice, that bi-directional navigability, according to both current and coming UML standards, is shown as line without arrows on both sides. Both-sided arrow is deprecated. THAT is the reason you won't see it often.

Can the composition itself be bi-directional? Can we see black diamonds on both sides of an association?

No , of course this sort of association cannot be mutual, for it is impossible for B to be created in A only and, simultaneously, for A to be created in B only.

What is interesting, the shared aggregation (empty diamond) cannot be mutual, too, but here the limitation is not inherent, it is simply forbidden by UML standard.

Answer 2

Yes, Composition does not add constraints with regards to the navigability of the association. More info on the difference between Accociation, Composition and Aggregations can be found here: UML Composition vs Aggregation vs Association

Answer 3

From https://www.lucidchart.com/pages/uml/class-diagram :

Bidirectional associations are the default associations between two classes and are represented by a straight line between two classes. Both classes are aware of each other and of their relationship with each other. In the example above, the Car class and RoadTrip class are interrelated. At one end of the line the Car takes on the association of "assignedCar" with the multiplicity value of 0..1 which means that when the instance of RoadTrip exists, it can either have one instance of Car associated with it or no Cars associated with it. In this case, a separate Caravan class with a multiplicity value of 0..* is needed to demonstrate that a RoadTrip could have multiple instances of Cars associated with it. Since one Car instance could have multiple "getRoadTrip" associations-- in other words, one car could go on multiple road trips--the multiplicity value is set to 0..*

Answer 4

In the past I had the same opinion as Gangnus with

So, can composition have bi-directional navigability?

But following some recent discussion I had a more detailed look into the UML specs. And simply, that statement is not true (only partially). Let's look into the UML 2.5 specs. On p.110 it is stated

Sometimes a Property is used to model circumstances in which one instance is used to group together a set of instances; this is called aggregation. To represent such circumstances, a Property has an aggregation property, of type AggregationKind; the instance representing the whole group is classified by the owner of the Property, and the instances representing the grouped individuals are classified by the type of the Property. AggregationKind is an enumeration with the following literal values:

[omitting shared aggregation]

composite: Indicates that the Property is aggregated compositely, ie, the composite object has responsibility for the existence and storage of the composed objects (see the definition of parts in 11.2.3).

Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.

Note my emphasis on the object/instance in the above text. So UML just talks of responsibility. If A composes B it will be responsible to delete B when it is destroyed itself. Vice versa B would be responsible for A 's destruction. So, if you have references in both directions (ie diamonds on both sides) then you will be reponsible to delete the object on the other side. This of course works only if just one of both holds a reference to the other. If both would have a reference, it would not be possible to have a directed responsibility (because it's circular).

I still think that having composite aggregation on both sides is not really a good idea. But according to the specification it is possible.