Class Instances: Objects

• While classes are abstractions, instances of classes (objects) make up a running object-oriented program.

• Objects are like virtual computers realized in software; they have both state (data) and behavior (operations).

• A main principle of OOP:

  A particular object is an instance of all the classes in its ancestry.

  Therefore, a particular Oxford shirt is an instance of the OxfordShirt and Item classes.

• This principle accounts for an operation's being "generic" (applicable to many types) or in the parlance of OOP, polymorphic.