Choose the Proper Screen-Based Controls 513

Step 7: Choose the Proper Screen-Based Controls 513

— Users may have difficulty recalling sufficient information to type entry, making text box unusable. — The list may be ordered in an unpredictable way, making it hard to find items.

Proper usage: — For entering or selecting objects or values or setting attributes. — For information that is mutually exclusive (only one can be entered or selected). — When users may find it practical to, or prefer to, type information rather than

selecting it from a list. — When users can recall and type information faster than selecting it from a list. — When it is useful to provide the users a reminder of the choices available. — Where data must be entered that is not contained in the selection list. — Where screen space is available. — For data and choices that are

• Best represented textually. • Somewhat familiar or known. • Ordered in an unpredictable fashion. • Frequently changed. • Large in number. • Variable or fixed in list length.

Description.

A combo box, also known as an attached combination box, is a single rec-

tangular entry field, beneath which is a larger rectangular box (resembling a drop-down list box) displaying a list of options. In Java, combo boxes are called editable choice pop-up lists. The entry field permits a choice to be keyed within it, while the larger box contains a list of mutually exclusive choices, from which one may be selected for placement in the entry field. A combo box combines the capa- bilities of both a text box and a list box. It visually resembles a drop-down list box or drop-down combo box (to be described). The text box and its associated list box have a dependent relationship. As text is typed into the text box, the list scrolls to the nearest match. Also, when an item in the list box is selected, that item is placed within the text box, replacing the existing content. When typing into the field, the information keyed does not have to match the list items. Combo boxes are illustrated in Figure 7.75.

Figure 7.75: Combo boxes.

514 Part 2: The User Interface Design Process

Advantages/disadvantages. Combo boxes are flexible, permitting selection or typed entry. Alternatives are always visible, or retrievable, reminding people of the available options. An unlimited variety of entries and choices are possible. Entries are not necessarily restricted to items selectable from a box. Combo boxes do con- sume quite a bit of screen space. Because all box choices may not be visible, some scrolling may be required. It is always possible that people may have difficulty recalling sufficient information to type, making the text box unusable. The list may also be ordered in an unpredictable way, making it hard to find items. Additional work is required of the user if selection scrolling must be performed.

Proper usage. Combo boxes are useful for entering or selecting objects or values or setting attributes that are mutually exclusive. They are most valuable when users may find it practical to, or prefer to, type information rather than selecting it from

a list, but where reminders of alternatives available must occasionally be pro- vided. They are also useful when the listings are dynamic and changeable, per- mitting the user to key items not contained on the list in the box. They do require that screen space be available to display them, but they eliminate the extra steps involved in retrieving drop-down lists. Combo boxes are useful for textual data and choices that are frequently changed and somewhat familiar or known, fos- tering keyed entry. The lists may be long, variable, and ordered in an unpre- dictable fashion.

Combo Box Guidelines For the text box entry field, see “Text Box/Single Line” guidelines. For the list box, see

“Drop-Down/Pop-Up List Boxes” guidelines.