A second basic building block for UML diagrams is a typed element. Typed elements (as you might expect from the name) have a type. Fields and parameters are typed elements, as are method parameters and return values.

The pattern for defining a typed element is:

$$ \texttt{[visibility] element : type [constraint]} $$

The optional $\texttt{[visibility]}$ indicates the visibility of the element, the $\texttt{element}$ is the name of the typed element, and the $\texttt{type}$ is its type, and the $\texttt{[constraint]}$ is an optional constraint.

Visibility

In UML visibility (what we would call access level in C#) is indicated with symbols, i.e.:

  • $\texttt{+}$ indicates public
  • $\texttt{-}$ indicates private
  • $\texttt{#}$ indicates protected

I.e. the field:

protected int Size;

Would be expressed:

$$ \texttt{# Size : int} $$

Constraints

A typed element can include a constraint indicating some restriction for the element. The constraints are contained in a pair of curly braces after the typed element, and follow the pattern:

$$ \texttt{ {element: boolean expression} } $$

For example:

$$ \texttt{- age: int {age: >= 0}} $$

Indicates the private variable age must be greater than or equal to 0.