Table of Contents
In all application domains, there are string-valued attributes with a fixed set of possible
string values. These attributes are called enumeration
attributes, and the fixed value sets defining their possible string values are called
enumerations. For instance, when we have to manage data about
persons, we often need to include information about the gender of a person. The possible values
of a gender attribute may be restricted to one of the enumeration labels "male","female" and
"undetermined", or to one of the enumeration
codes "M", "F" and "U". Whenever we deal with codes, we also need to have
their corresponding labels, at least in a legend explaining the meaning of each code.
Instead of using the enumeration string values as the internal values of an enumeration
attribute (such as gender), it is preferable to use a simplified internal
representation for them, such as the positive integers 1, 2, 3, etc., which enumerate the
possible values. However, since these integers do not reveal their meaning (which is indicated by
the enumeration label) in program code, for readability we rather use special constants, called
enumeration literals, such
as MALE or M, prefixed by the name of the enumeration, like
GenderEL, in program statements like this.gender = GenderEL.MALE.
Notice that we follow the convention that the names of enumeration literals are written all upper
case, and that we also use the convention to suffix the name of an enumeration data type with
"EL" standing for "enumeration literal" (such that we can recognize from the name
GenderEL that each instance of this datatype is a "gender enumeration
literal").
There are also enumerations having records as their instances, such that one of the record fields provides the name of the enumeration literals. An example of such an enumeration is the following set of units of measurement:
Table 8.1. Representing an enumeration of records as a table
| Units of Measurement | ||
|---|---|---|
| Unit Symbol | Unit Name | Dimension |
| m | meter | length |
| kg | kilogram | mass |
| g | gram | mass |
| s | second | time |
| ms | milisecond | time |
Notice that since both the "Unit Symbol" and the "Unit Name" fields are unique, either of them could be used for the name of the enumeration literals.
In summary, we can distinguish between the following three forms of enumerations:
simple enumerations define a set of self-explanatory enumeration labels;
code lists define a set of code/label pairs.
record enumerations consist of a set of records, so they are defined like classes with simple attributes defining the record fields.
These three forms of enumerations are discussed in more detail below.
Notice that, since enumerations are used as the range of enumeration attributes, they are considered to be data types.
Enumerations may have further features. For instance, we may want to be able to define a new enumeration by extending an existing enumeration. In programming languages and in other computational languages, enumerations are implemented with different features in different ways. See also the Wikipedia article on enumerations.
A simple enumeration defines a fixed set of
self-explanatory enumeration labels, like in the example of a GenderEL enumeration
shown in the following UML class diagram:
Since the labels of a simple enumeration are being used, in capitalized form, as the names
of the corresponding enumeration literals (GenderEL.MALE,
GenderEL.FEMALE, etc.), we may also list the (all upper case) enumeration literals
in the UML enumeration data type, instead of the corresponding (lower or mixed case) enumeration
labels.
A code list defines a fixed set of code/label pairs. Unfortunately, the UML concept of an enumeration data type does not support the distinction between codes as enumeration literals and their labels. For defining both codes and labels in a UML class diagram in the form of an enumeration data type, we may use the attribute compartment of the data type rectangle and use the codes as attribute names defining the enumeration literals, and set their initial values to the corresponding label. This approach results in a visual representation as in the following diagram:
In the case of a code list, we can use both the codes or the labels as the names of
enumeration literals, but using the codes seems preferable for brevity (GenderEL.M,
GenderEL.F, etc.). For displaying the value of an enumeration attribute, it's an
option to show not only the label, but also the code, like "male (M)", provided that there is
sufficient space. If space is an issue, only the code can be shown.
A record enumeration defines a record type with a unique field designated to provide the enumeration literals, and a fixed set of records of that type. In general, a record type is defined by a set of field definitions (in the form of primitive data type attributes), such that one of the unique fields is defined to be the enumeration literal field, and a set of operation definitions.
Unfortunately, record enumerations, as the most general form of an enumeration data type, are not supported by the current version of UML (2.5) where the general form of an enumeration is defined as a special kind of data type (with optional field and operation definitions) having an additional list of unique strings as enumeration literals (shown in a fourth compartment). The UML definition does neither allow designating one of the unique fields as the enumeration literal field, nor does it allow populating an enumeration with records.
Consequently, for showing a record enumeration in a UML class diagram, we need to find a workaround. For instance, if our modeling tools allows adding a drawing, we could draw a rectangle with four compartments, such that the first three of them correspond to the name, properties and operations compartments of a data type rectangle, and the fourth one is a table with the names of properties/fields defined in the second compartment as column headers, as shown in the table below.
Table 8.2. Representing a record enumeration as a table
| UnitEL | ||
|---|---|---|
«el» unitSymbol: String unitName: String dimension: String |
||
| Unit Symbol | Unit Name | Dimension |
| m | meter | length |
| kg | kilogram | mass |
| g | gram | mass |
| s | second | time |
| ms | milisecond | time |