On Wikipedia, I'm reading that an association relationship is an instance level relationship so we are talking about the relationship between the objects of two classes.
When we actually draw a class diagram, why do we use association on the class elements or blocks rather than objects? And there are also class level relationships for which we again use class elements. Since we don't have any way to show if we are talking about objects or classes I find this confusing. For example: I've heard people saying "Associate these two classes" Doesn't that sound wrong?
Links are to Associations as Objects are to Classes.
A Class is an abstraction that describes many specific objects. Similarly, an Association is an abstraction that describes many links between objects.
So your statement
an association relationship is an instance level relationship
isn't strictly correct because it mixes the abstraction (Association Relationship) with the instances it represents.
hth.
In fact, when you associate two or more classes it is done thanks to two or more UML Properties.
These latter are the "ends" of your associations and are "instance" i.e. they are typed by classe.
So an association is created between two classes on a class diagram but between each classes and the association you have a UML property.
Hoping it sounds clear ...
When you are making a class diagram you are defining types. Suppose you have a class User and a class Account, you use an association between User and Account to say: User instances can have link(s) with Account instances at runtime.
So, you use classes and associations at type level (class diagram) to define what can be possible at runtime (instance level).
The object is actually the Class that has been created virtualy. So a class is the "static" version of an Object. So, when we speak of UML, we speak about classes and not object.
But correct me if I'm wrong!
Related
Hello I'm trying to prepare an ER diagram + a class diagram.
The system is very simple. It involves a simple flight reservation system with two roles (actors).
In the ERD, we can distinguish between the two roles using the "IsAdmin" attribute("true" for admin, and "false" for customer).
In the class diagram, I have a base class named "User" (containing all the attributes and methods of the customer role and all the attributes and some of the methods of the admin role).
Should I use the User class when dealing with the customer entity (since the attributes and methods are the same), or should I create an empty class derived from the User class for clarity?
It depends...
Basically when dealing with business objects I would not use a class named "User" which says all or nothing. You have a "Customer" and an "Admin" and likely many other "User"s. Don't make the "User" a "Customer".
Setting that aside, if you are using a serialization framework that often requires table and class to have the same name.
Also you shouldn't start a system with optimization. Give clarity an advantage in favor of that. It will pay out more sooner than later. And in any case: you can optimize at later stages as well.
ER Diagrams (ERDs) and UML Class Diagrams (CDs) are both information modeling languages that essentially cover the same concepts, but using a different visual syntax. Both allow modeling classes with properties/attributes (and operations/methods in the case of CDs) defining entity types, and associations (or relationship types).
Since CDs have been defined more recently, they are more expressive than, and subsume, ERDs. Consequently, there is no need to duplicate an information model both as an ERD and a CD. Rather, you should make a logical class design diagram from which you can obtain both OOP class models (defining, e.g., Java classes) and RDB table models.
If needed, you can derive an ERD from your CD.
In a UML diagram where one class is comprised by another class, is there an implicit association table, or does it need to be shown?
For example, if I have Chapters that are composition of Paragraphs, is this alone enough:
Or does the association class to be explicitly shown like so:
I've never seen it done like that latter example, so I'm assuming it's implied. Or maybe I haven't normalized the data properly (considering chapter both appears in the Chapter class and the Paragraph class).
Simply, there is no implication. If you need an association class, you need to define it. However, UML is not about diagraming, it's about modeling. You can omit your association class in a diagram if you want to. The association class will still exist in your model, though.
No, it is not implied.
The reason why you haven't seen this is that in most cases it simply doesn't make sense. In the association class the class part additionally describes the properties of the association. In your example you create an artificial object that actually brings little or nothing. This kind of approach can be useful for many-to-many relationships which are impossible for composition (each part can have at most one whole). Even if you wanted to show Foreign Key it would simply be on one of the classes. But that's implied by the association itself. According to UML specification an inline attribute is equivalent to an association.
Moreover on UML you can depict many to many relationship simply but applying appropriate multiplicities on association ends. You may want to show the class depicting that only when modelling on the code level.
No, it is not implied.
UML is not about tables, but about classes. The author of the class diagram should tell the audience how a class should be interpreted. Some UML diagrams model the real world, others model a piece of application functionality and yet others depict a physical implementation.
Apparently, reading your question, you are modeling a relational database, where a class is a physical table. In that case, I would expect that every table is explicitly modeled as a UML class.
The UML standard does not demand this.
By the way, the notation (PK) and (FK) is not in accordance with the UML standard.
Some classes in my class diagram need to reference specific instances of other classes or themselves. Should I just model those the same way i would in an object diagram or is there a better option? In general: Can one combine class and object diagrams, since object diagrams are themselves class diagrams?
thanks!
UML2 introduced Composite Structure Diagram exactly to solve this issue.
In a Composite Structure Diagram you can show classifiers (e.g. Classes) together with theirs internal composition in terms of instances.
This way you can specify exactly which instances are linked to your objects.
See this article for a good explanation.
I agree with #Thomas Killian : you want to mix two different representations.
On the class diagram you will be able to show cardinalities, but not instances relationships. It seems your List class is not a simple list but a chain / linked list.
A ListElement is in fact part of a LinkedList. Two predecessor and successor attributes themselves of type ListElement (or how you wish to call this class, the node role could be noted) will be enough to suggest the behavior. As an attribute has a 1:1 cardinality with the encapsulating class, this respects what you wish to model. Renaming the list class would be a hint for the diagram reader.
Take a look at the Java Linked List source if you want some ideas: this could help you to design a clear class structure, the initial author is not so bad at classes design. In this case, he pushes encapsulation to the excess but the idea is right.
Can I improve my design on these 2 diagrams below? If so, how?
I am confused by the use of association classes in my diagrams. Should I use them?
Figure 1
Figure 2
For association classes, they make sense only if they represent associations with some behaviour and state. Look at Article'sSubject. It has nothing more than source, target and identity. You don't need a class for such a association, just use plain association, which has all those properties. There are more such unnecessary association classes in your diagrams. Another important thing when using association classes is that every instance of that association class should have unique source and target pair. For example Comment is not identifiable just by Article and User - one user may have man ycomments on one article and that is not allowed.
An association class is used to capture certain characteristics of an association between two classes. These characteristics do not belong to the classes being associated but instead belong to the relationship between the classes.
Sorry for this newb question, i'm new to UML.
The diagram for a system is this one:
From what I know of UML, none of the classes in this diagram can own instances of the associated class as there's no aggregate relationship with it.
Does this mean in an implementation of the system in Java, based on the diagram, an outside class has to own instances of the associated class?
Sorry if the answer is obvious. I've spent hours scratching my head over it.
First off, terminology. #Daniel is right, you don't have an association class. However, I don't think you mean Association Class:
Does this mean in an implementation of the system in Java, based on the diagram, an outside class has to own instances of the associated class?
If I understand correctly that's the nub of your question. In implementation terms, which class(es) have a member variable containing a list of references to instances of Associated Class?
Again - if I understand right - your question stems from the following logic:
In UML, "ownership" is commonly described as a quality of Aggregation (or Composition) relationships.
The relationship between Aggregated/Composite PART Class and Associated Class is a simple binary association - not Aggregate/Composite.
Therefore the "ownership" property doesn't apply
Therefore who owns the list of references to Associated Class instances?
If that's right then the issue is with the specific meaning of "ownership". Whilst not tightly defined in UML, "ownership" typically means responsibility for managing full lifecycle.
I think you're interpreting it more generally: that if an association isn't aggregate, then the participating classes can't hold references to each other.
That's not the case. It's perfectly reasonable for Aggregated/Composite PART Class to hold a reference (or list of references) to instances of Associated Class. The inverse is equally valid. In some cases both are valid (with the attendant need to maintain consistency).
So in summary: is it necessary for an outside class to own the instances of Associated Class? No. It's perfectly valid for either or both ends of a binary association to manage instances of the relationship.
hth and apologies if I misunderstood your question.
PS: a final observation: be very careful about what you mean when using Aggregation. It's notoriously imprecise in the UML spec. Composition has a more rigorous definition, and you can cover 99% plus of all modelling scenarios using Composition and plain Binary Associations. About the only place Aggregation has a well-defined meaning not completely covered by the other two is denoting when recursive relationships must be acyclic.
UML does not specify the full behaviour of a system. So what do you mean, when you say an object owns another object? Also instances AssociatedClass could be root objects that are not owned by any other object.
The diagram you provided doesn't really contain an association class. The class you named 'associated class' is just a normal class. It also isn't owned by anything (that we see in the diagram).
If what you had in mind was association class, then take a look at example diagram with association class:
In this example, the MilleageCredit is an association class. So for each distinct combination of Fligh-FrequentFlyer there is one MilleageCredit.
As for ownership, since the Association class represents a relation between 2 associated objects, it gets deleted when
the association is cleared
either or both of associated objects are deleted
So if you delete either the Flight or FrequentFlyer the MilleageCredit will be gone too.
Also if you unlink Flight from FrequentFlyer again the MilleageCredit will be delete.
There's plenty of good UML docs online, for example UML basics: The class diagram
Hope this helps, otherwise please provide more info in the question.