1.1 Objects and classes

Objects

In the "real" world, objects are the entities of which the world is comprised. Everything that happens in the world is related to the interactions between the objects in the world. Just as atoms, which are objects, combine to form molecules and larger objects, the interacting entities in the world can be thought of as interactions between and among both singular ("atomic") as well as compound ("composed") objects. The real world consists of many, many objects interacting in many ways. While each object may not be overly complex, their myriad of interactions creates the overall complexity of the natural world. It is this complexity that we wish to capture in our software systems.

In an object-oriented software system, objects are entities used to represent or model a particular piece of the system.

Objects are the primary units used to create abstract models.

There are a number of schools of object-oriented programming, which differ slightly on how they view objects. Here, we will take a "behaviorist" (our term) stance:

An object is characterized solely by it behaviors.

This does not mean however, that an object may not contain data (information) in fields. The essence of the object is in how the object behaves in relationship to or as a result of its data, should it contain any.

The existence of data in an object is an implementation technique used to generate the required behavior of that object.

Classes

Many objects differ from each other only in the value of the data that they hold. For example, both a red crayon and a blue crayon are crayons; they differ only in the value of the color attribute, one has a red color and the other a blue color. Our object-oriented system needs a way to capture the abstraction of a crayon, independent of the value of its color. That is, we want to express that a set of objects are abstractly equivalent , differing only in the values of their attributes and perhaps, thus differing in the behaviors that result from those values.

Many objects are similar in many overall, generalized ways, differing only in smaller, more specific details. In biology and other fields, scientists organize objects into taxonomies, which are classification hierarchies used to express these similarities. For instance, a butterfly and a lobster are quite different, yet they share the common characteristics of all Arthropods, such as a jointed exoskeleton. The notion of an Arthropod enables us to understand and deal with butterflies and lobsters in an abstract, unified manner. So once again we are trying to express abstract equivalence.