We have learned that the modeling of phenomena by means of objects represents properties in terms of e.g. data-items and methods, and we have learned how classes of objects represent the corresponding domain concepts. Specialization hierarchies are expressed by a class being a subclass of another (super) class. As an example the class Customer from before could have been defined as a subclass of a class Person, and Person would have a name attribute of type String.
class Person:
name: var String
class Customer: Person
addr: var String
email: var StringAltogether this model the situation that a customer is a person and that a customer has the properties represented by data-items and methods. A Customer object is a Person and it has the properties of Person and the properties defined in Customer. We have also seen that an object (obj) also may have a super class.
In this section, we will show to model the situation where phenomena in addition to be classified (and thereby represented by a class) also has some aspects. An aspect is a set of properties that may readily be modelled by data-items, methods, and activities. Many phenomena may have the same aspects, and this calls for the definition of types of aspect. As an aspect may have the same properties as an object, types of aspects may be represented by classes. It will therefore be possible to classify types of aspects.
+++ faktaboks Most other languages have interfaces that represent aspects, and interface types and subtypes. Most interfaces only have method signatures, while some languages, e.g. C# support that methods of interfaces may have behaviour descriptions. Mechanisms like mixin classes an traits have been proposed for handling aspects. Users of so-called multiple inheritance (i.e. that a class may have multiple superclasses) either use superclasses for adding aspects or simply in order to reuse code from a number of classes. We shall see that our aspects cover all of these cases.
In most cases, classes/objects with aspects will also be subclasses of classes from the domain, e.g. Customer being a subclass of Person. As we only have tree-structured classification by means of classes and subclasses, i.e. a class can only have one superclass, aspects come in play when a class in addition to a superclass also shall have the properties defined by one or more aspect classes.
In the following we will, by a number of examples, show how to represent types of aspects by classes, and to use composition to represent aspects of classes and objects. Composition means that an object with aspects simply has an object for each aspect. An aspect object is specified as an object of an aspect class or as a singular object with an aspect class as superclass.
A class like Customer may typically tailor the fact that it is a Person, by extending possible virtual methods of class Person. We will have the same need for aspects. An aspect may e.g. be that of being addressable. Different objects and classes may be addressable, and each of them may have a slightly different way of being addressable. We will therefore show how to tailor a general type of aspect to specific objects and classes.
