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:

[visibility] element: type [constraint]

The optional [visibility] indicates the visibility of the element, the element is the name of the typed element, and the type is its type, and the [constraint] is an optional constraint.

Visibility

In UML visibility (based on access modifiers in Java, or the use of underscores in Python) is indicated with symbols, i.e.:

  • + indicates public access.
  • - indicates private access.
  • # indicates protected access, which we will discuss in a later chapter.
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Consider, for example, a private size field. In a Java class, we would do the following:

Java
private int size;

Consider, for example, a private size field. In Python, we might have the following assignment in our constructor:

Python
self.__size: int = 0;

In a UML diagram, that field would be expressed as:

- 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:

{element: boolean expression}

For example:

- age: int {age: >= 0}

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