CSCI383 ObjectOriented Programming Design Lecture 10 Reflexive Associations

CSCI-383 Object-Oriented Programming & Design Lecture 10

Reflexive Associations • It is possible for an association to connect a class to itself

Directionality in Associations • Associations are by default bi-directional • It is possible to limit the direction of an association by adding an arrow at one end • In a specification view this would indicate that a Day has a responsibility to tell which notes it is associated with, but a Note has no corresponding ability to tell which day it is associated with • In an implementation view, one would indicate, that Day contains pointer(s) to Note(s), but a Note would not point to any Day

Generalization • Specializing a superclass into two or more subclasses. They must follow the isa rule • Our idealization of inheritance is captured in a simple rule-ofthumb. Try forming the English sentences ``An A is-a B''. If it “sounds right” to your ear, then A can be made a subclass of B – A dog is-a mammal, and therefore a dog inherits from mammal – A car is-a engine sounds wrong, and therefore inheritance is not natural but a car has-a engine

Generalization • Represented using a small triangle pointing to the superclass – The discriminator is a label that describes the criteria used in the specialization

Generalization • Generalization captures similarities between several classes in a superclass. Specialization refines and adds differences in subclasses

Inheritance is both Extension and Contraction • Because the behavior of a child class is strictly larger than the behavior of the parent, the child is an extension of the parent (larger) • Because the child can override behavior to make it fit a specialized situation, the child is a contraction of the parent (smaller)

Avoiding Unnecessary Generalizations Inappropriate hierarchy of classes, which should be instances

Handling Multiple Discriminators What about sharks?

Handling Multiple Discriminators • Creating higher-level generalization The platypus, a difficult case for single inheritance Why? Egg-laying mammal • Results in duplication of features • Another possible solution is multiple inheritance. However, it adds too much complexity and is not supported by many OOP languages (e. g. , Java, C++)

Avoiding Having Instances Change Class • An instance should never need to change class • How to solve this problem?

More Advanced Features: Aggregation • Aggregations are special associations that represent ‘partwhole’ relationships – The ‘whole’ side is often called the assembly or the aggregate – This symbol is a shorthand notation association named is. Part. Of

More Advanced Features: Aggregation • Example: “A CPU is part of a computer” • Example: “A car has an engine and doors as its parts”

When to Use an Aggregation • As a general rule, you can mark an association as an aggregation if the following are true – You can state that • the parts ‘are part of’ the aggregate • or the aggregate ‘is composed of’ the parts – When something owns or controls the aggregate, then they also own or control the parts

When to Use Aggregation • Aggregation vs. attributes – Attributes describe properties of objects (e. g. , speed, price, length) – Aggregation describe assemblies of objects • Aggregation vs. association – Is a company an aggregation over its employees or is it an association between its employees?

Composition • A composition is a strong kind of aggregation – if the aggregate is destroyed, then the parts are destroyed as well – A one-to-one composition often corresponds to a complex attribute. Two alternatives for addresses (i. e. , as an attribute or as a composition)

Interfaces • An interface is a (abstract) class with no implementation – An interface is implemented (refined) by (different) classes – Example: A portable text editor displays its windows using a window interface that is implemented differently for Windows 95 and Mac OS

Abstract Classes • An abstract class is a class without a (full) implementation – Some methods are deferred (i. e. , they are not implemented) – The deferred methods are implemented by subclasses only – Example: The window move operation is implemented by using hide and show methods which are implemented by subclasses

Example Class Diagram EXAMPLE: UML class diagram for airline reservation system (done in class)

How to Use Class Diagrams • Class diagrams are the backbone of nearly all objectoriented methods. Especially they facilitate code generation • The trouble with class diagrams and their rich notation is that they are extremely detailed and therefore confusing – Do not try to use all the notations available to you, if you do not have to – Don’t draw diagrams for everything; instead concentrate on the key areas