I've started using the website draw.io, I want to create a class diagram but I couldn't find how to create an association class, any help would be useful. Thanks.
draw.io is a drawing tool, not a UML tool.
As such you can simply draw two class shapes, connect them with an association shape, then draw a third class as you association class and then use an association shape to connect you association class to the association.
Then go into the properties of the last association shape and change the line style to dashed.
Now it looks like a valid UML Association class, but I think you should really opt for a real UML tool instead of a drawing tool when you want to do anything with UML. Using this tool is similar to writing Java with MS Word.
The accepted answer is far to be perfect. In Draw.io (now called Diagrams.net), you can connect two shapes (here classes) with a connector but you cannot connect two connectors together.
Consequently, with the given solution, it will look like an association class but the dashes will not be really adjusted :
And really worste, if you need to re-arrange your diagram, the association link will not move and you will have to re-adjust it manually :
So, as it is sadly not possible to have a 100% satisfying solution for an association class, another solution is to turn the association class as a regular class. Here in the example, the OrderLine class is associated with both Order and Product classes. It is worth to note the migration of the multiplicities. Refactoring a class association as such an association will ALWAYS lead to have a 1 multiplicity for the non-association classes (here Order and Product), and the original multiplicities will be moved to the association class (here OrderLine).
Related
I have recently been studying UML and drawing simple diagrams with ordinary plain arrows between classes, but I know it's not enough. There are plenty of other arrows: generalization, realisation and etc. which have meaning to the diagram reader.
Is there a nice resource which could explain each arrow (ordinary, plain, dotted, diamond-filled, diamond)?
It would be the best if it will have some code examples for them.
Here's some explanations from the Visual Studio 2015 docs:
UML Class Diagrams: Reference: https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2015/modeling/uml-class-diagrams-reference
5: Association: A relationship between the members of two classifiers.
5a: Aggregation: An association representing a shared ownership relationship. The
Aggregation property of the owner role is set to Shared.
5b: Composition: An association representing a whole-part relationship. The Aggregation
property of the owner role is set to Composite.
9: Generalization: The specific classifier inherits part of its definition from the general
classifier. The general classifier is at the arrow end of the connector. Attributes, associations, and
operations are inherited by the specific classifier. Use the Inheritance tool to create a
generalization between two classifiers.
13: Import: A relationship between packages, indicating that one
package includes all the definitions of another.
14: Dependency: The definition or implementation of the dependent classifier might change if
the classifier at the arrowhead end is changed.
15: Realization: The class implements the operations and attributes defined by the interface.
Use the Inheritance tool to create a realization between a class and an interface.
16: Realization: An alternative presentation of the same relationship. The label on the
lollipop symbol identifies the interface.
UML Class Diagrams: Guidelines: http://msdn.microsoft.com/library/dd409416%28VS.140%29.aspx
Properties of an Association
Aggregation: This appears as a diamond shape at one end of the connector. You can use it to
indicate that instances at the aggregating role own or contain instances of the other.
Is Navigable: If true for only one role, an arrow appears in the navigable direction. You can use
this to indicate navigability of links and database relations in the software.
Generalization: Generalization means that the specializing or derived type inherits attributes,
operations, and associations of the general or base type. The general type appears at the arrowhead
end of the relationship.
Realization: Realization means that a class implements the attributes and operations specified by
the interface. The interface is at the arrow end of the connector.
Let me know if you have more questions.
I think these pictures are understandable.
A nice cheat sheet (http://loufranco.com/wp-content/uploads/2012/11/cheatsheet.pdf):
It covers:
Class Diagram
Sequence Diagram
Package Diagram
Object Diagram
Use Case Diagram
And provides a few samples.
My favourite UML "cheat sheet" is UML Distilled, by Martin Fowler. It's the only one of his books that I've read that I do recommend.
For quick reference along with clear concise examples, Allen Holub's UML Quick Reference is excellent:
http://www.holub.com/goodies/uml/
(There are quite a few specific examples of arrows and pointers in the first column of a table, with descriptions in the second column.)
The accepted answer being said, It is missing some explanations.
For example, what is the difference between a uni-directional and a bi-directional association? In the provided example, both do exist. ( Both '5's in the arrows)
If looking for a more complete answer and have more time, here is a thorough explanation.
A very easy to understand description is the documentation of yuml, with examples for class diagrams, use cases, and activities.
Aggregations and compositions are a little bit confusing. However, think like compositions are a stronger version of aggregation. What does that mean?
Let's take an example:
(Aggregation)
1. Take a classroom and students:
In this case, we try to analyze the relationship between them. A classroom has a relationship with students. That means classroom comprises of one or many students. Even if we remove the Classroom class, the Students class does not need to destroy, which means we can use Student class independently.
(Composition)
2. Take a look at pages and Book Class.
In this case, pages is a book, which means collections of pages makes the book. If we remove the book class, the whole Page class will be destroyed. That means we cannot use the class of the page independently.
If you are still unclear about this topic, watch out this short wonderful video, which has explained the aggregation more clearly.
https://www.youtube.com/watch?v=d5ecYmyFZW0
If you are more of a MOOC person, one free course that I'd recommend that teaches you all the in and outs of most UML diagrams is this one from Udacity: https://www.udacity.com/course/software-architecture-design--ud821
Just a simple question: I'm working on a class diagram for a dynamic web site for my internship. I have an association between two classes (client and article), the relation is many to many.
Normally I have to add an association class between them, but I don't want to because the client can only read the article's writing on the web page, and he cannot post. Is it correct if I don't draw the association class on the diagram?
There is no need to add association class only due to many to many relationship. The association class is needed when you have an association that at the same time is also a class, i.e. has some properties on its own.
You can use simple association with any multiplicity on any of the ends including having * on both ends.
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!
Okay so I'm a bit confused as to when I should add an attribute to a class vs drawing an association to a class when trying to show a relationship in a UML diagram.
For example let's say I have a DFA class that contains 10 state objects each having a different set of paths to various states in the DFA. Should I draw the composition line from the DFA to the State class or just type all 10 states in the attribute section of the DFA class.
Basically I'm trying to figure out if when a Class A contains (or is composed of) one or more Class Bs, should I draw a line (aggregation, composition,association dependency etc) between them or just put one as an attribute to another.
This article attempts to answer the question but I get confused as soon as he starts talking about the diagrams he drew.
The article you pointed gives one guideline:
In the years I’ve been working with different modelling teams I’ve found that the rule that works best is to use Associations for Classes and Attributes for DataTypes.
A data type is a special kind of classifier, similar to a class. It differs from a class in that instances of a data type are identified only by their value.
I kind of agree with the author, adding another point: your class diagram should be understandable. If you have a diagram with many lines crisscrossing each other the diagram is useless. When this happens you must try to find relations that can be modeled using attributes without loosing too much visual information, and replace them with attributes.
Another thing that you can do with relations and not with attributes is to display bi-directional relations (you can do this with one attribute in each class of the relation, but you loose the semantics of the relation).
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.