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Given this pseudo code:

class B { }

class A : B
    int val;

alpha = new A();

What arrow do I draw between alpha and A in a UML class diagram? Is this even something UML is meant to do?

+-------+        +-------+        +-------+
| alpha | --??-- |   A   |------|>|   B   |
|       |        |       |        |       |
+-------+        +-------+        +-------+

In my case, I truly do want to express explicitly that alpha instantiates A. So if there is no way in UML, I'll just make up a way -- which is fine, but I want to know if UML can express it already. My situation is that I'm trying to explain OOP to a friend who is very visual. Therefore, I want to try to visually distinguish the two relationships (inheritance and instantiation) so that he can distinguish it mentally.

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up vote 10 down vote accepted

In a class diagram, you don't show the act of instantiation. That is something that a sequence or communication diagram would show. Class diagrams show the static structure of a system.

If a class A "depends on" a class B, then you could look at the association and dependency relationships. Dependency is the weaker of the two and is usually used if the other class is a parameter to a method and is not stored as an instance variable. Association is stronger and means that class A contains an instance (or instances) of class B, and is further strengthened by the composition and aggregation relations.

In your example, alpha isn't part of a class. If alpha was an instance variable in some class C, you would connect C to A with an association. If it was a parameter to a method or created inside a method of C and not stored as an instance variable, you would use a dependency line to connect C to A. In all cases, you have the ability to specify directionality and multiplicity.

I think a larger problem is that you appear to be associating UML with "class diagram". The UML 2.2 specification has 14 diagram types. There are 7 structural diagrams, and 7 behavioral diagrams (broken down into 3 behavior diagrams and 4 interaction diagrams). Each diagram shows a particular view of the system. Some aspects of the system are best shown using a particular diagram, or in some cases, a series of diagrams. If you are interested, I would recommend checking out Wikipedia's entry on UML and Martin Fowler's UML Distilled. Both give great overviews of the purpose, strengths, and weaknesses of various diagram types.

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I agree with @Thomas Owens that class diagrams in UML are not meant to show instances in general. However, I must add that UML does provide mechanisms to depict instantiation:

  • the InstanceSpecification element provides a way to model instances, such as alpha in your example.
  • the dependency notation (a dashed arrow with plain arrowhead), flowing from the instance to its type, and adorned with the stereotype <<instanceOf>>, provides a way to model the instantiation relationship between an instance and its classifier(s).
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Are you referring to what is discussed on I've never seen the instanceOf stereotype, but I have seen <<creates>> and <<uses>> before. – Thomas Owens Aug 15 '11 at 17:12
@Thomas Owens: Well, <<instanceOf>> is a well known stereotype since the earliest versions of UML. If you look at the latest stable spec ( of the UML infrastructure, you'll find multiple examples of its usage, such as the classical Figure 7.8 about the four-layer metamodel hierarchy that has been used ad nauseam in a multitude of publications. – CesarGon Aug 15 '11 at 17:27
Interesting. I'll have to poke around. Although I'm familiar with UML, I've never dug into the formal spec and all of it's little intricies before. – Thomas Owens Aug 15 '11 at 17:36
@Thomas Owens: Sure, and good luck. :-) – CesarGon Aug 15 '11 at 18:24
To add a reference: – Alexander Bird Aug 16 '11 at 11:15

enter image description here

There is a special stereotype for such dependency. Class INSTANTIATES Class2, if it creates its instances. Exists in UML standard. ( I checked 2.4.1 - contemporary and 2.5 - the next one.)

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