当A类与B类具有聚合和组合关系时，如何绘制UML图？

Question

I have two questions to ask. So let's assume there's a class A and B which are defined like these.

1.

class A {
    private B b;
    private B otherB;
    public A(B otherB)
    {
        this.otherB = otherB; 
    }
}
class B {
}

So class A has a composition relationship with the variable b and aggregation relationship with the variable otherB. How can I draw this in UML diagram.

2.Would the following case be still a composition relationship?

class A
{
    private B b;
    public B getMethod(){
        B newB = new B();
        newB.bValue = b.bValue;
        return newB;
    }  
}
class B
{
    private int bValue;
}

Answer 1

1. As other comments/replies have pointed out, there is not problem of having different associations (composed or not) between the same classes.

2. From the implementation point of view (and this also applies to the previous question) you need to understand what a composition association means .

Basically, if we have instance specification a1 and a2 (as instances of the class A ), only one of them could compose an instance b1 (as instance of the class B ) via the role (association end) "composesB" of the composite association.

Likewise, provided that a1 composes b1 via the "composesB" role of a composite association, everytime a1 gets "destroyed", b1 should also be "destroyed". Instead, this wouldn´t occur if a1 object aggregated b1 via the "aggregatesB" role of an aggregate association.

As you may imagine, from an implementation point of view, you need much more than a field and a simple method in a class in order to support a composite association between two classes.

Update: to include an example.

For example, EMF is an implementation of the EMOF specification (it´s not UML) in which the concept of a containment reference (similar to the concept of a composite association) can be depicted as follows. In our particular case:

Going away from technical details, you may grasp that when you set a B instance as part (contained, composed) by an A instance object. You firstly have to check that the former might be contained in a different A instance via the same containment reference, if so, such B instance needs to be removed from the old A instance one.

Answer 2

1. Creating Multiple Associations Between Classes is, in fact, legal. The recommendation is to assign a role to each relationship when you do so.

2. I'm going to say yes, that is still a composition relationship (though others may disagree). To make my case, we look at the simple IBM definition showing Student composed of Schedule (presumably amongst other things). It's very easy to imagine that there would also be a Teacher composed of Schedule, and that a Teacher might want to get the Schedule of a Student, or vice versa. Does the getting of a Schedule by another class invalidate the composition relationship? I think not; or at least, the IBM definition does not appear to be so narrow as to preclude that possibility.