How to represent in UML that subclasses of an abstract class should be singleton

Question

Suppose an abstract class X and its subclasses Y and Z. How do I represent in UML class diagrams that Y and Z should be singletons. Is it possible to represent that all X subclasses must be singletons?

Answer 1

To specify that all subclasses of X are singletons, you can write a constraint in between braces: { every subclass of X is a singleton }. This constraint should be put in a constraints compartment in the class rectangle.

The UML 2.5 specification, §7.6.4 defines the notation for constraints in general and §9.2.4 specifies how to show the constraints of a classifier:

If a Classifier owns Constraints, a conforming tool may implement a compartment to show the owned Constraints listed within a separate compartment of the owning Classifier's rectangle. The name of this optional compartment is “constraints.”

Alternatively, you could give a singleton indication on each and every subclass of X. From your wording, I assume that that is not what you want. Anyway, the latest version of UML (2.5.1) does not have a standard way to indicate that a class is a singleton. Some people indicate it by writing 1 in the top right corner of the rectangle. However, that is not valid UML. You may use that for parts, but not for classes. Instead, you could invent your own stereotype ≪singleton≫.

There is another StackOverflow question about this topic .

Answer 2

§11.4.4 of the UML 2.5 spec says:

A usage dependency may relate an InstanceSpecification to a constructor for a Class, describing the single value returned by the constructor Operation. The Operation is the client, the created instance the supplier.

If you create a GeneralizationSet that has the meta-property isComplete=true (to say that all possible subclasses are accounted for), and you connect one InstanceSpecification to each constructor by a usage dependency, the model means that every class is a singleton.

Answer 3

Here's another possibility: you can adorn the class with a <<singleton>> stereotype. I always used it that way and the coder knows how to handle that. It's no UML standard, but see the last sentence.