Is it safe to say that in a UML class diagram, if there is an association of n to 1 objects, then the association arrow is pointing from the class associated with n objects to the class associated with 1 object?
n ----> 1
Not in general. The arrow direction in UML corresponds to navigation possiblities. Hence, if your arrow points from the n objects to the 1 object it means that in your resulting code each of the n objects can access that 1 object. Of course you may also want an association in which that 1 object can access the n objects, so the arrow points in the different direction. (If no arrowhead is given it means that the n objects can access the 1 object and vice versa.)
Not precisely. It depends on the type of association. If its an aggregation then the default is bi directional navigability, but you can specify the navigation to be one way. I think that it is entirely up to the designer to decide the direction of the navigation between objects.
No. At least in object oriented programming, the one class that has the "1" association (reference in OOP) could equally well have the many references to the other class.
It depends on the design you make.
I thought the arrow was more about navigability (i.e. an arrow from A -> B shows that class A knows how to get to B but B does not know how to get back to A) than expressing the cardinality of the relationship.
If I can suggest, there are 3 different notions:
1 - Navigability which is modeled by arrow
2 - Cardinality which is modeled by number or star for 0..n
3 - to who belongs the end point of the association which is modeled by a dark point
please see extracted from the norm.
Related
I have this class/object diagram:
I don’t understand why that object diagram is invalid according to the given class diagram.
In the object diagram, one C object has two links with two T objects, alpha relationship with a T object and beta relationship with another T. So I don’t think it violates the multiplicity constraints.
Could you please explain me why the object diagram is invalid?
Yours is the most interesting question I've seen here in a long time. It is pretty tricky!
The simple reason your instances are incorrect is that every instance of type T must be associated with one C. The top instance of type T in your diagram violates the constraint in association beta. (The multiplicity on the left end of the association.)
There are two faults in the object diagram.
There is only a formal fault in the object diagram, the lines in the objects diagrams between the instances are links, i.e., instances of the associations shown in the class diagram. As the links are instances, the same rules for instance naming apply as to class instances. So change alpha to :alpha and underline it, it is correct. Same for beta.
Further the links are not correct, as there is an beta link from the uppermost T instance missing. Each object of A, and as C is a specialization of A, also C (and B) objects need an alpha link to an S instance. As S is a generalized T, an alpha link between A (or one of its specializations) and S (or one of its specializations) is needed. Further each S (or T) might have arbitrary alpha links to A objects.
Each C object needs to have zero or one beta links to T instances. In the other direction, each T instance needs exactly one C instance via a beta link. This is missing for the uppermost T instance.
Leaving my prior answer below, but thinking twice, the answer is that your class diagram is incomplete.
The two alpha and beta associations have no association-end names. The fact that they have different multiplicities leads to the conclusion that they must be different associations. With names it would look like this:
Expanding the inheritance will make this:
Based on this assumption, my original answer stands like this:
The reason is that a :C has two associations alpha and beta each to another :T object. Not a single alpha to one and a single beta to another. So you need to add a beta to the alpha and vice versa.
Edit And yes, JimL. is correct. Having two alpha-links violates the constraint from the class diagram. So actually you can only have one T linked to C. Which again makes the class model very strange.
The C class has a beta-association to T. C inherits from A and T inherits from S. Since there is a alpha-association from A t0 S this is also inherited. So you have:
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.
:)
What is the differences between property owned by an association and a property owned by a class in UML?
is there a simple example helps me understand the differences?
The difference is more conceptual than anything else. If you have a property attached to an association, then you will have an association class, with the desired property.
Here is an example of a mailman sending letters to clients (the attribute weight is bound to the association):
The difference is very concrete, but traditionally ignored.
A is associated to B. A,B are classes.
If B end of the association is "association owned" and navigable, it means, that you can easily reach instance of B from A, either by reference, or by some method(s). It should be shown by arrow.
If B end is "classifier owned", you know a bit more. it means, that A has an attribute, that is a direct reference. (No functions or reference counting here). It should be shown by arrow and "dot"- a small black filled circle.
If you are going to B by a.smth.smthelse.b, it is arrow, but surely no dot.
If you are going to B by a.b, it is arrow and dot.
If you haven't decided yet, it is arrow again.
Traditionally modellers show arrows only. But it is not a good tradition, and is against UML standard. Diagrams are more useful if we decide as much as possible on them, not in code.
If both ends are navigable, both arrows disappear and you may see only dot(s).
The association lines with cross with dot on one side, or arrow on one side and only dot on the other are senseless.
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.
What are are the meanings of symbols marked 1,2 & 3 ?
The first one is aggregation.
The second one is a connector for the comment
The thirs is a dependency.
1) aggregation (special form of association)
2) this is simply the connector for the comment
3) generic dependency
1) the open diamond means weak form of aggregation. This relationship denotes that
the aggregate class (the class with the white diamond touching it) is in some way the “whole”, and the other class in the relationship is somehow “part” of that whole.
2) Similar notation is used to represent an interface, but I don't think this is the case here, ergo - I don't know what it is.
3) The dashed arrow is a dependency relationship or ‘using’ relationship. This relationship simply means that ConcreteBuilder somehow depends upon Product. This is very weak relationship and is not implemented with member variables at all.