In general, in object-oriented programming, a class is an abstract definition of both (1.) data types and (2.) actions that can be performed on that data. The latter are commonly known as object methods.

While classes only describe an object type in an abstract way, the objects that really exist in a program are called instances of a certain class.

Take a WPS program object, for example: every program object is an instance of the class WPProgram, defined by the WPS. The (1.) data of the program object would be, among others, the executable associated with it (e.g. "CMD.EXE"), while (2.) methods (that could be performed on this data) would be starting the executable (by double-clicking or choosing the "Open" menu item) or changing the executable in the settings notebook. While you cannot "see" methods from the user interface, methods are used all the time internally every time the WPS is doing something. If you're familiar with that term, methods can vaguely be described as an Application Programmer's Interface (API) to an object. If you know how to program REXX: every time you use one of those object setup strings to modify an object's data, you're actually calling a method upon an object.

Since methods are defined for each class, the methods that you can call on an object depend on the class of the object. I have not counted them, but already WPObject, the most basic WPS class, defines more than 100 methods.

What makes object-oriented programming so flexible (and popular) is that you can define relationships between classes. In order to understand this, the following two concepts are helpful:

• Inheritance. This concept allows you to define an object class that takes over all its data types and methods from another object class, without requiring that the programmer know how the original class was actually programmed. You can simple "take over" the features of an existing class and rewrite only what you wish to implement. The original class is then called the "ancestor" or "parent" of the "descendant" class.

  For example, with the Workplace Shell, the Desktop class (called "WPDesktop") is derived from the folder class (called "WPFolder"), making the Desktop just a special type of folder.

  So basically, the Desktop has all the capabilities of a regular folder: you can put other objects into it, sort its contents, open a tree or details view, and so on. But in addition, the Desktop class provides a few more actions: its context menu contains more items (such as "Shutdown" and "System configuration"), its notebook has more pages, and so on. Obviously, the Desktop inherits folders' characteristics, but adds a few more. On the other hand, some folder characteristics are suppressed: for example, you cannot close the active Desktop, and it does not have a title bar.

  The WPS implements a derivative structure such that all objects are logically grouped and interdependent. On an even more abstract level, all object classes of the WPS are descendants of a SINGLE CLASS named "WPObject", which provides certain functions that all Desktop objects need: mainly the ability to display context menus and notebooks at all and things like that. This is typical of object-oriented programming; such a global ancestor class is commonly called the "root class" of a "class hierarchy".

  On WPObject, the WPS builds a complete tree of classes known as the WPS class hierarchy. You can use the "WPS Classes" page in &xwp;'s "WPS class list" object to take a look at this.

  Such a class hierarchy has the advantage that one can change the characteristics of this root class only, and all classes which are derived from it will also change. (The disadvantage is -- from a programmer's point of view -- that building such a class hierarchy requires a whole lot of planning which data and which methods are common to which classes, and many times it turns out that this planning wasn't quite optimal. But once this hierarchy has been built in a thoughtful way -- which, thanks to IBM, indeed is the case with the WPS -- the advantages are just superb.)
  

• Polymorphism. This concept means that, while using inheritance, you can REPLACE certain characteristics of an ancestor class with new ones. You can redefine ("override") the original object class's behavior. Now, if a certain method is performed on the descendant, the newly defined method is called instead of the one that was defined with the ancestor class.

  Just one of the many examples of this can be seen with the "Help" menu item which is available in every object's context menu. Selecting the "Extended help" menu item will result in an object's wpDisplayHelp method to be called. This method is introduced by the by the WPS root class, WPObject, so that all Desktop objects can display help: the &os2; help manager is initialized, the help window is displayed, etc. The actual help page to be displayed however is overridden (following the concept of polymorphism) by almost every single WPS class. This way, pressing "Help" in a folder's context menu will show you something different than from, say, in a program object's menu.

This only works because the WPS uses IBM's own System Object Model (SOM), a complex system which provides object-oriented programming interfaces even across several code modules, and even independent of the programming language. SOM is so powerful because classes are instantiated and maintained at run-time instead of compilation time.

This is, by the way, where the WPS class list comes in: when the WPS is booting up, it creates all the classes (which, in SOM, are objects too, but this is a complex issue), and establishes the relationships between them only at this point. Only because of this is it possible to modify classes without IBM, who created the originals, even knowing about it.

Just as a short personal opinion, this is still something that makes &os2; unique in today's computer world. While most of its other advantages over other operating systems, such as its reliable multitasking, are not that unique any more (e.g. Linux is pretty good at that, too), the user interface is still not reached by any operating system that I know. Especially not Windows 95.