An action which is suffered by, or required of, an object. Operations may be selectors, constructors, or iterators. An operation is contained in an o… - Edward V. Berard

Work on what was to become “structured design” began in the early 1960s. Structured design, as a well-defined and named concept, did not achieve appreciable visibility until the publication of an article in the IBM Systems Journal in 1974 (Stevens et al., 1974). There is more than one way to accomplish a “structured design.”

An object which is used to create instances, i.e., a template, description, pattern, or "blueprint" of a category or collection of very similar items. Among other things, a class describes the interface the these items will present to the outside world, i.e., the available and appropriate methods, constants, and exceptions. A class represents an abstraction of the items. A class may itself be parameterized (i.e., it actually represents a family of very closely related classes), in which case we refer to it as a parameterized class. Class is a recursive concept. Specifically, we may define classes as being composed of other classes (i.e., heterogeneous composite classes and homogeneous composite classes), in terms of itself (a recursively defined class), as inheriting characteristics from one or more other classes (i.e., the superclasses of the class), and as providing characteristics to other classes (i.e., the subclasses of the class). In some places, classes are defined as "the set of all instances of a type," and the term "type" is given the above definition for class.

Recently, I have had conversations with a number of people who were attempting their first object-oriented project. Most of these people claimed to be using “use cases” in their object-oriented analysis and design approaches. However, listening to them describe their projects, it became quite apparent to me that there is a great deal of confusion regarding the definition, context, and use of use cases. This confusion can (and often does) lead to poorly-designed systems.