Luciano García Bañuelos - ut · 22 Class modeling -- Luciano García-Bañuelos Part can be shared by several wholes Part is always a part of a single whole Parts can live independently

Class modeling

Luciano García Bañuelos Institute of Computer Science

Object versus Class

Class modeling -- Luciano García-Bañuelos 1

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A class describes a group of objects with the same properties,

behavior, kinds of relationships, and

semantics

An object corresponds to the description of a single entity/instance in the application domain

Class diagram


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Some details can be omitted

Richer specifications are also possible

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Class associations


  In the application domain, objects are usually linked together

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Object links

Class association

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Multiplicity and end names


Multiplicity specifies the number of objects that can be related with respect to

their underlying class

Examples •  1 (default) •  * multiple •  1..* one or more •  3..5 three to five, inclusive

An association end name specify the role of a class in the association

employee employer

More about associations


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A pair of classes can have multiple associations

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Association classes


Association may also have properties

giving rise to association classes

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Association classes may participate in other

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Generalization


  Generalization is a relationship between classes providing an organized view of possible variations in both structure and behavior

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Generalization and specialization


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Generalization

Specialization

The superclass holds common properties

and associations

Each superclass adds specific properties and

associations

Inheritance and overriding


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&"'"(!"#$%Inheritance is the mechanism for sharing attributes, operations, and associations via the generalization/specialization relationship

A subclass overrides a superclass operation when it lists the same name. The overriding

operation is expected to refine and/or replace the

overridden operation.

Generalization set name


  The generalization set name is an optional attribute that indicates the aspect being abstracted by a particular generalization

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The class Vehicle can be specialized according to the following aspects:

•  Means of propulsion: wind, fuel, animal, gravity •  Operating environment: land, air, water, outer space

Class properties: Advanced modeling


Multiplicity •  [1] (default) •  [*] multiple •  [1..*] one or more •  [3..5] three to five, inclusive

Visibility (depends on the programming language used for implementation)

•  + public •  # protected •  - private •  ~ package

Constraints for multi-valued attributes

•  {ordered} •  {unique}

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Navigation


  If an association is directed, messages can pass only on that direction

  If the association does not have directions, then it is a bidirectional association

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Dependencies


  A dependency is the most general relation between classes   It indicates that an object affects another object

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Qualifiers


  A qualifier is an attribute or list of attributes whose values serve to partition the set of objects associated with an object across an association

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A qualifier selects among the target objects, reducing the effective multiplicity from “many” to “one”

Qualifier

Constraints


  Constrains are simple properties of associations, classes and many other things in UML

  Specify limitations that implementers need to satisfy

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Examples of constraints


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Only one association can be instantiated

The ChairOf association is a subset of MemberOf

association

Tasks must be ordered

Salary constraint must be enforced by the

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Constraints in UML


  Constraints can be applied to almost every element in UML diagrams, using:   natural language   mathematical notation   OCL (Object Constraint Language)

  Constraints can be used for expressing:   Invariants

  interest > 3%   Preconditions

  before loan() takes place, salary > 5,000$   Postconditions

  after loan() takes place, dayCollect = 1 or 10

N-ary associations


  Occasionally, you will find some associations among three or more classes   Try to decompose those n-ary associations into binary associations

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The meaning of the new model might be slightly different (for instance, you must be careful about multiplicity)

Aggregation and composition


  Aggregation is a special form of association   Underlines the fact that an object is made of constituent parts

  Composition is a more restrictive form of aggregation   Two additional constraints

  A constituent part can belong to at most one assembly   The part has a coincident lifetime as the assembly

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Association versus Composition


Part can be shared by several wholes

Part is always a part of a single whole

Parts can live independently (i.e., whole cardinality can be 0..*)

Parts exist only as part of the whole (e.g. when a Window is destroyed all other widgets are also destroyed)

Whole is not solely responsible for the object

Whole is responsible and should create/destroy the objects

0..4 Category Document

*Window Frame

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Abstract classes


  An abstract class is a class that has no direct instances   It may define common properties   It may define some operation signatures called abstract operations

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An abstract class is specified with a name in an italic font (or using

{abstract} near the class name)

Abstract operation (use an italic font)

Multiple inheritance


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A class can specialize more than

one superclasses

Be careful with the “diamond problem”:

some overlapping may cause conflicts

The “diamond problem” will not be present if the

classes are disjoint

{disjoint, incomplete}

Multiple inheritance


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In this example: •  overlapping There might be vehicles that belong to more than one subclass •  complete Although more combinations might exist, only those presented are meaningful

UML generalization set constraints:

{complete, disjoint} {incomplete, disjoint} default {complete, overlapping} {incomplete, overlapping}

Delegation as a workaround for multiple inheritance


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Metadata


  Metadata is data that describes other data   Database catalogs are an example of metadata   This a central concept to disciplines such as the Model Driven

Architecture (MDA)

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Enumerations


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An enumeration is a data type that has a finite set of values. You should avoid modeling enumerations as generalization hierarchies.

Derived data


  A derived element is a function of one or more elements, which in turn can be derived

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Packages


  A package is a group of elements (classes, associations, generalizations and nested packages) with a common theme   A model can be partitioned in packages to make it easier to

understand

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Luciano García Bañuelos - ut · 22 Class modeling -- Luciano García-Bañuelos Part can be shared by several wholes Part is always a part of a single whole Parts can live independently