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A more formal model of this abstraction can be easily

achieved by following an object-oriented (OO) approach:

All information is contained in numerous interrelated

information items (or objects) of different information

forms (or classes). Each information form can be formally

specified by a class definition and each information item

can be considered as an instance of a class following this

class definition. The attributes section of a class definition

defines a template for the possible content, relations and

metadata of the individual information items and the

methods section of the class definition specifies all possible

ways of interacting with the item or manipulating it.

This object-oriented model comes in handy when

considering Jones and Teevan’s observation that although

“a person’s interactions with an information item vary

greatly depending upon its form” “there are many essential

similarities in the way people interact with information

items, regardless of their form.” By using the OO

mechanism of inheritance it is possible to model such

essential similarities between different information forms in

a common base class. If properly applied this leads to a

class hierarchy of information forms which integrates very

different types of information while preserving a maximum

degree of consistency in attributes and methods. At first

glance this might only appear interesting for programmers.

However, such a consistent class hierarchy can also have a

strong impact on usability and UI design when following

the approach of “Object-oriented User Interfaces”.

Application-Oriented

User Interfaces

Object-Oriented User

Interfaces

Application consists of an

icon, primary window,

and secondary windows

Product consists of a

collection of cooperating

objects and views of objects

Icons represent running

applications

Icons represent objects that

may be directly

manipulated

Users must start

application before

working with objects

Users open objects into

views

Provide users with

function needed to

perform a task

Provide users with supplies

needed to perform a task

Rigid structure-by

function

Flexible structure-by object

Users must follow the

application structure

User may perform in their

own way or innovate

Many applications

required – one per task

Few objects – more reuse of

the same object in many

tasks

Table 1. Application-oriented UI vs. OOUI (taken from [27]).

Object-oriented user interfaces (OOUIs) were introduced to

the PC GUI in connection with IBM’s OS/2 operating

system in the early 1990s. At that time OS/2’s object-

oriented approach was perceived as the direct competitor to

the application-orientation in the GUI of Microsoft

Windows and was regarded as more “intuitive” and

“natural” by many HCI researchers [27]. However, due to

the rapidly growing popularity of the Windows operating

system and the advent of the hyperlink-driven World Wide

Web, OOUIs have never gained much attention and

momentum in UI design or PIM since then.

Figure 1. Domain model and data sources of a ZOIL-based UI.

One of the key concepts of OOUIs is to deploy functiona-

lity as a collection of cooperating objects and views of

objects which are directly integrated as new components

into a common visual workplace. Instead of creating

monolithic applications running in isolated windows with a

specialized interaction model, rigid workflows and non-

interoperable data formats, new functionality is seamlessly

integrated into the existing desktop environment. This

preserves a high degree of consistency and interoperability

on the UI (see table 1 from Theo Mandel [27]). Unlike in

monolithic applications, tasks in OOUIs are then carried out

in the desktop environment by direct manipulations in a

very flexible “model world” interface instead of indirect

interactions in specific applications with rigidly structured

“conversational interfaces” and dialogs [18]. Therefore

OOUIs appear especially suitable for such complex and

unstructured “knowledge work” [10] like PIM where

workflows and tasks are highly user-specific and cannot be

easily automated or supported by conventional dialogs.

Only fragments of the original OOUI principles have found

their way into today’s desktop metaphor and applications,

e.g. the object-action sequence of context/pop-up menus or

the “pick, drag and drop” interaction with icons. ZOIL

takes up OOUI concepts in their original sense to provide

the user with a flexible though consistent direct

manipulation interface for PIM.

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