TYPES OF USER INTERFACES IN SOFTWARE ENGINEERING

 

9.1 TYPES OF USER INTERFACES

Broadly speaking, user interfaces can be classified into the following three categories:

 Command language-based interfaces  Menu-based interfaces

 Direct manipulation interfaces

Each of these categories of interfaces has its own characteristic advantages and disadvantages. Therefore, most modern applications use a careful combination of all these three types of user interfaces for implementing the user command repertoire. It is very difficult to come up with a simple set of

guidelines as to which parts of the interface should be implemented using what type of interface. This choice is to a large extent dependent on the experience and discretion of the designer of the interface. However, a study of the basic characteristics and the relative advantages of different types of interfaces would give a fair idea to the designer regarding which commands should be supported using what type of interface. In the following three subsections, we briefly discuss some important characteristics, advantages, and disadvantages of using each type of user interface.

9.1.1 Command Language-based Interface

A command language-based interface—as the name itself suggests, is based on designing a command language which the user can use to issue the commands. The user is expected to frame the appropriate commands in the language and type them appropriately whenever required. A simple command language-based interface might simply assign unique names to the different commands. However, a more sophisticated command language-based interface may allow users to compose complex commands by using a set of primitive commands. Such a facility to compose commands dramatically reduces the number of command names one would have to remember. Thus, a command language-based interface can be made concise requiring minimal typing by the user. Command language-based interfaces allow fast interaction with the computer and simplify the input of complex commands.

Among the three categories of interfaces, the command language interface allows for most efficient command issue procedure requiring minimal typing. Further, a command language-based interface can be implemented even on cheap alphanumeric terminals. Also, a command language-based interface is easier to develop compared to a menu-based or a direct-manipulation interface because compiler writing techniques are well developed. One can systematically develop a command language interface by using the standard compiler writing tools Lex and Yacc.

However, command language-based interfaces suffer from several drawbacks. Usually, command language-based interfaces are difficult to learn and require the user to memorise the set of primitive commands. Also, most users make errors while formulating commands in the command language and also while typing them. Further, in a command language-based interface, all interactions with the system is through a key-board and cannot take advantage of effective interaction devices such as a mouse. Obviously, for

casual and inexperienced users, command language-based interfaces are not suitable.

Issues in designing a command language-based interface Two overbearing command design issues are to reduce the number of primitive commands that a user has to remember and to minimise the total typing required. We elaborate these considerations in the following:

 The designer has to decide what mnemonics (command names) to use for the different commands. The designer should try to develop meaningful mnemonics and yet be concise to minimise the amount of typing required. For example, the shortest mnemonic should be assigned to the most frequently used commands.

The designer has to decide whether the users will be allowed to redefine the command names to suit their own preferences. Letting a user define his own mnemonics for various commands is a useful feature, but it increases the complexity of user interface development.

 The designer has to decide whether it should be possible to compose primitive commands to form more complex commands. A sophisticated command composition facility would require the syntax and semantics of the various command composition options to be clearly and unambiguously specified. The ability to combine commands is a powerful facility in the hands of experienced users, but quite unnecessary for inexperienced users.

9.1.2 Menu-based Interface

An important advantage of a menu-based interface over a command language-based interface is that a menu-based interface does not require the users to remember the exact syntax of the commands. A menu-based interface is based on recognition of the command names, rather than recollection. Humans are much better in recognising something than recollecting it. Further, in a menu-based interface the typing effort is minimal as most interactions are carried out through menu selections using a pointing device. This factor is an important consideration for the occasional user who cannot type fast.

However, experienced users find a menu-based user interface to be slower than a command language-based interface because an experienced user can

type fast and can get speed advantage by composing different primitive commands to express complex commands. Composing commands in a menu- based interface is not possible. This is because of the fact that actions involving logical connectives (and, or, etc.) are awkward to specify in a menu- based system. Also, if the number of choices is large, it is difficult to design a menu-based interfae. A moderate-sized software might need hundreds or thousands of different menu choices. In fact, a major challenge in the design of a menu-based interface is to structure large number of menu choices into manageable forms. In the following, we discuss some of the techniques available to structure a large number of menu items:

Scrolling menu: Sometimes the full choice list is large and cannot be displayed within the menu area, scrolling of the menu items is required. This would enable the user to view and select the menu items that cannot be accommodated on the screen. However, in a scrolling menu all the commands should be highly correlated, so that the user can easily locate a command that he needs. This is important since the user cannot see all the commands at any one time. An example situation where a scrolling menu is frequently used is font size selection in a document processor (see Figure 9.1). Here, the user knows that the command list contains only the font sizes that are arranged in some order and he can scroll up or down to find the size he is looking for. However, if the commands do not have any definite ordering relation, then the user would have to in the worst case, scroll through all the commands to find the exact command he is looking for, making this organisation inefficient.

 

Figure 9.1: Font size selection using scro ling menu.

Walking menu: Walking menu is very commonly used to structure a large collection of menu items. In this technique, when a menu item is selected, it

causes further menu items to be displayed adjacent to it in a sub-menu. An example of a walking menu is shown in Figure 9.2. A walking menu can successfully be used to structure commands only if there are tens rather than hundreds of choices since each adjacently displayed menu does take up screen space and the total screen area is after all limited.

 

Figure 9.2: Example of walking menu.

Hierarchical menu: This type of menu is suitable for small screens with limited display area such as that in mobile phones. In a hierarchical menu, the menu items are organised in a hierarchy or tree structure. Selecting a menu item causes the current menu display to be replaced by an appropriate sub-menu. Thus in this case, one can consider the menu and its various sub- menu to form a hierarchical tree-like structure. Walking menu can be considered to be a form of hierarchical menu which is practicable when the tree is shallow. Hierarchical menu can be used to manage large number of choices, but the users are likely to face navigational problems because they might lose track of where they are in the menu tree. This probably is the main reason why this type of interface is very rarely used.

9.1.3 Direct Manipulation Interfaces

Direct manipulation interfaces present the interface to the user in the form of visual models (i.e., icons 2 or objects). For this reason, direct manipulation interfaces are sometimes called as iconic interfaces. In this type of interface, the user issues commands by performing actions on the  visual  representations  of the  objects, e.g., pull  an icon

representing a file into an icon representing a trash box, for deleting the file.

Important advantages of iconic interfaces include the fact that the icons can be recognised by the users very easily, and that icons are language- independent. However, experienced users find direct manipulation interfaces very for too. Also, it is difficult to give complex commands using a direct manipulation interface. For example, if one has to drag an icon representing the file to a trash box icon for deleting a file, then in order to delete all the files in the directory one has to perform this operation individually for all files

—which could be very easily done by issuing a command like delete *.*.