Let's say we have a supermarket chain and a number of supermarkets belong to it.
If the chain ceases to exist the supermarkets will have to shut down. This could be implemented in UML using a composition relationship. On what side of the association should the filled-in diamond be located?
Should it be like Chain----<.>supermarket ? Or chain<.>-----supermarket) ?
Should I think of it as the supermarkets belong to the chain or the chain has supermarkets?
In UML, you put the composition diamond next to the class that composes another. In your example, An individual of type Chain composes [1] several individuals of type Supermarket, so the black diamond goes on the Chain end of the association.
[1] Some people pronounce this "Chain has Supermarkets", which is indistinguishable from a different sense of the word "has", as in "Chain has name".
Related
While I googled to understand the Unary associations, I got the following two explanations:
the first is:
A unary relationship is when both participants in the relationship are
the same entity. For Example: Subjects may be prerequisites for other
subjects, or one employee manages many Employees.
and the second is:
Class B knows about ClassA.
Class A does not know about ClassB.
Now lets look at the following example:
You can see the Person and Address relationship below. We call this
relationship as has-a relationship since person has a address. So
Person knows the address but address does not know anything about
person
Am I misunderstanding something?
Common language
The arity of an association is about how many classes are associated. This is an ambiguous concept since some understand different classes, whereas others understand instances.
When applied to unary, the first interpretation would mean reflexive association (or self-association, i.e. a class associated with iteself), whereas the second would mean a class associated with nothing (not very useful: any class could be associated with nothing else).
UML perspective
Fortunately, the UML specifications are much more precise than the common language:
An Association specifies a semantic relationship that can occur between typed instances. It has at least two memberEnds represented by Properties, each of which has the type of the end. More than one end of the Association may have the same type.
So in UML there is no "unary association". It's binary, ternary, or n-ary (terms used in the specs). There is no special term in UML for a binary association with the same class at both ends. But reflexive or self-association are terms which are more popular than unary.
E/R modeling
The term "unary" is popular in the context of entity-relationship modeling, to describe a relation in a relational database. Relations correspond more or less to an association in UML, and entities to classes, but there are some subtle semantic differences. E/R has its foundation in the set theory. And if a relation is between the same entities, it means in fact that only one set is involved. This is probably why unary is more popular in this context.
Merriam-webster defines Unary as
having, consisting of, or acting on a single element, item, or
component
So the first explanation is correct one since this type of association acts on a single class. The term Unary however is not used in UML and might be confusing.
UML uses the term binary to indicate that an association has two ends, and ternary or n-ary to indicate an association has multiple ends.
The Unary association you are talking about is actually a binary association to itself, also known as reflexive association.
Not to be confused with a Unidirectional association, which is an association that is only navigable (has an arrow) to one side.
Considerer this:
As far as I know in case 1 a is related to b, and a is aware of b so a is able to send messages to b, but b is not aware of a so b is not able to send messages to a.
In the other hand, in case 2 a is related to b, and both of them are aware of each other, both of them are able to send messages to the other.
But my question is what about that when the relationship is not association but aggregation? Could an aggregation relationship have bidirectional navegability in UML so both instances are aware of each other?
In case a owns b, consequently a is aware of b. (in other words a is able to send messages to b). but what about the other way around, does b is aware of a? or does this could never be the case? or this is just not specified in the diagram and this could be both ways? and, in case b has to be aware of a how would that be expressed(I've never seen an aggregation line with an arrowhead in the other end)?
does all of this applies in the same way for composition?
Another thing slightly related to this I Was wondering, could a non bi-directional association have two roles?
As far as I know this could not be possible because a non bi-directional relationship is a relation when just one of the related instances is aware of the other, and a role means how an instance perceive another one, so we need that both instances are aware of each other to be able to have two roles, is this correct?
Directed associations as per UML 2.5:
A DirectedRelationship represents a relationship between a collection of source model Elements and a collection of target model Elements.
So this does not say much. And in fact you can simply leave it away. There is a concept of non-navigability which explicitly rules out navigation.
Ownership is not shown by an arrow but by a dot near the owning class.
Another cite from Superstructures (p. 200 in chap. 11.5 Associations):
Navigability notation was often used in the past according to an informal convention, whereby non-navigable ends were assumed to be owned by the Association whereas navigable ends were assumed to be owned by the Classifier at the opposite end. This convention is now deprecated. Aggregation type, navigability, and end ownership are separate concepts, each with their own explicit notation. Association ends owned by classes are always navigable, while those owned by associations may be navigable or not.
If you specify a role name that explicitly means you have navigability towards the named class.
I have one object, call it type A which has four data members of another object type, call it B. How do I show this in a UML class diagram so that its clear there are four B objects type in every A object?
Is the only solution to put "4" next to the arrow head pointing to class B?
It depends on what you want to achive, in sense of how you need to distinguish between those objects in context of their association/link, that is - what kind of role they play:
if there are all equal, no special differences in their role in context of A, them a multiplicity 4..4 will do the job, naming the association end properly (for example my_Bs)
If these object play different role in connection with A, then you can use separate associations with lower multiplicities each one, 2, 3 or even 4 pieces (for example, if B is a Wheel and A is Car, then you can put 2 associations with multiplicities 2..2 each, and call then "front" and "rear", or even 4 associations "front_left", "front_right"...)
Here is how the both cases look like. On the second one I showd different possible options (with max. 5 elements of B), just to give you an idea.
It's probably clear by now, but the fundamental concept here is the role of the association end.
Aleks answer is the best. However you can also represent the multiplicity in one box like this :
You cal also use composite structure diagram. See example below:
From my point of view, myBs defined as an attribute of type B on class A has a different meaning of myBs defined as a association's role between A and B (which is also different as defining it as a composition/aggregation).
If it is an attribute, then it's not a role. In that case, there is only a simple dependency relation between A and B, which must appear in the diagram.
I think that problem comes from unconsciously think from a "Relationnal Data (BMS)" and/or a "Object Oriented Programming" point of view, but UML is not intended for that.
:)
An association defines a semantic relationship between classifiers. The instances of an association are a set of tuples relating instances of the classifiers. Eachtuple value may appear at most once. The Association represents a set of connections among instances of the Classifiers. An instance of an Association is a Link, which is atuple of Instances drawn from the corresponding Classifiers
I wonder if there is someone helps me understand every word of the association definition especially the highlighted ones?because I read about it from different resources but all of them say the same words but I would like a more elaborated definition
semantic relationship
This means there's a structural relationship between the things being associated that arises from the problem space. For example: the association Person owns Dog. In a dog licensing application, this relationship is the central concept; the application exists to manage the links between people and dogs. It's a 'semantic' relationship because it has meaning which originates from the problem space.
set of tuples relating instances of the classifiers
A tuple is 'an ordered set of elements' (wikipedia). An example of the Dog-Ownership association could be ("Fido", "Fred") where "Fido" represents a Dog and "Fred" a Person. An association can be represented as a set of tuples in that there is one tuple for each combination of Dog & Person for which the relation holds; e.g.
[("Fido", "Fred"), ("Angel", "Chuck Norris"), ("Boatswain", "Lord Byron")]
Note there are no tuples for pairs where the relationship doesn't hold; e.g. ("Fido", "Lord Byron").
each tuple value may appear at most once
It's not possible for the set to contain duplicates as this would just be saying the same thing twice. So there's no point adding ("Fido", "Fred") again to the list above; we already know Fred owns Fido.
The Association represents a set of connections among instances of the Classifiers
This is just another way to think about the relationship. For each tuple in the set, you can think of a link - or connection - between the related objects.
An instance of an Association is a Link, which is a tuple of Instances
See above. Each tuple represents one linked pair of objects. Links are to Associations as Objects are to Classes. Classes have many objects; Associations have many Links.
Fundamentally associations exist to show where things are systematically linked to other things. Tuples and sets are a way to think about and/or represent those linked things. (In fact I'd quibble somewhat with the definition in your OP: the links in an association can be represented as as a set of tuples: but that's not what they are, it's how they're modelled. The same information could equally be modelled by a Graph, where each object was represented by a vertex (node) and each association an edge.
hth.
EDIT:
Responding to your questions. Looks like you understand it pretty well; some observations.
First, here's how I would model it:
Now to each of your points:
Name: is the name of Association relationship(optional,you can give it a name or not)
I prefer verb phrase based naming as it brings out the meaning of the relationship. My model can be read directly as:
Each Person owns many Dogs (where 'many' means 0 or more)
Each Dog is owned by exactly one Person
Doing so removes the need to name the association explicitly, although you can still do so if you want.
visibility(I am not care about it,at least for now, I didn't realize its importance until now).
I would agree. Personally, I never annotate models with visibility.
Name:(here is the name of MemberEnd ),so,I left its default name in the screenshot
See comment about association naming above. I prefer verb-based naming to role-based: 'owns' is much more explicit in describing the purpose of a relationship than naming the association end 'dog' or 'dogs'.
the owner of memberEnd [...]
Personally: I don't use this. There's a whole other discussion about this that tbh I don't believe has a material impact in most cases.
Navigable [...]
Again I don't use this personally. In reality navigability should be derived from the underlying behaviour. Does it require navigating one way/both? Then set navigability accordingly. However some people like to specify it explicitly, on basis it makes the implementation clearer (If only navigable one way it can be implemented with reference(s) in one class only; if bi-directional it needs references in both directions - with attendant logic to keep things consistent).
Multiplicity
I agree with your selection.
Hope that helps.
When creating relationships in an Object Class Diagram for an Object Relational Database, should the diamonds on the ends of the relationship links be filled in or not.
Here is an image of my Class diagram:
http://canning.co.nz/Weltec/Class_Diagram.png
It's a choice between Composition and Aggregation, which Wikipedia explains quite well.
In practice though, I think a valid answer is to just not worry about the difference, unless it's a school assignment. I've found that trying to make very detailed UML diagrams isn't terribly useful in practice.
Composition relationship has a coloured diamond shape structure ending at a class if it belongs to a 'is a' relationship (i.e- the entity cannot exist without the parent class) whereas the aggregation has an empty diamond shape ending at a class if it belongs to a 'has a' relationship (i.e- the entity can exist without the existence of the parent class.
A full diamond denotes Composition, or a 'owns' relationship. You use it when the referenced entity can't exist without the class representing it. An example would be order to order item. The order item just doesn't make sense without the order.
An empty diamond denotes Aggregation, or a 'has' relationship. A quick glance at your diagram makes me think this is the correct diagram element to use in your case.
But I agree with #mpartel: If there aren't any specific requirements to distinguish between the two just ignore the diamonds.