An enumeration attribute is an attribute that has an enumeration as its range.

In the user interface, an output field for an enumeration attribute would display the enumeration label, rather than its internal value, the corresponding enumeration index.

Figure 6.2. A single and a multiple select element with no selected option

For allowing user input to an enumeration attribute, we can use the UI concept of a (drop-down) selection list, which may be implemented with an HTML select element, such that the enumeration labels would be used as the text content of its option elements, while the enumeration indexes would be used as their values. We have to distinguish between single-valued and multi-valued enumeration attributes. In the case of a single-valued enumeration attribute, we use a standard select element. In the case of a multi-valued enumeration attribute, we use a multi-select element with the HTML attribute setting multiple="multiple".

In the case of using a single-select element for an optional enumeration attribute, we need to include in its options an element like "---" for indicating that nothing has been selected. Then, the UI page for the CRUD use case "Create" shows "---" as the initially selected option.

For both cases, an example is shown in Figure 6.2. While the single select element for "Original language" shows the initially selected option "---" denoting "nothing selected", the multiple select element "Other available languages" shows a small window displaying four of the options that can be selected.

For usability, the multiple selection list can only be implemented with an HTML select element, if the number of enumeration literals does not exceed a certain threshold (like 20), which depends on the number of options the user can see on the screen without scrolling.

Figure 6.3. A radio button group

For user input for a single-valued enumeration attribute, a radio button group can be used instead of a single selection list, if the number of enumeration literals is sufficiently small (say, not larger than 7). A radio button group is implemented with an HTML fieldset element acting as a container of labeled input elements of type "radio", all having the same name, which is normally equal to the name of the represented enumeration attribute.

Figure 6.4. A checkbox group

For user input for a multi-valued enumeration attribute, a checkbox group can be used instead of a multiple selection list, if the number of enumeration literals is sufficiently small (say, not larger than 7). A checkbox group is implemented with an HTML fieldset element acting as a container of labeled input elements of type "checkbox", all having the same name, which is normally equal to the name of the represented enumeration attribute.