Object modeling

Object Modeling

OOAD
MCA 405 C (N2)

Amandeep S. Patti 1

Objects and Classes
 Object can be a concept, abstraction or
thing with crisp boundaries and meaning
for the problem at hand.
 They promote understanding of the real
world and provide a practical basis for
computer implementations.

Amandeep S. Patti 2

Objects and Classes
 A class describes a group of objects with
similar properties (attributes), common
behaviour (operations), common relations
to other objects and common semantics.
 The objects in a class share a common
semantic purpose.

Amandeep S. Patti 3

Object Diagram
 Object diagram provide a formal graphic
notation for modeling objects, classes and
their relationships to one another.
 Object diagrams are useful both for
abstract modeling and for designing actual
programs.

Amandeep S. Patti 4

Object Diagram
 There are two types of object diagrams.
 A class diagram is a schema, pattern or
template for describing many possible
instances of data. A class diagram describes
object classes.
 An instance diagram describes how a
particular set of objects relate to each other.
An instance diagram describes object
instances.

Amandeep S. Patti 5

Attributes
 An attribute is a data value held by the objects in
a class.
 Each attribute has a value for each object
instance.
 Different object instances may have same or
different values for a given attribute.
 Each attribute name is unique within a class.
 An attribute should be a pure data value, not an
object.

Amandeep S. Patti 6

Operations and Methods
Operations
 An operation is a function or transformation that

may be applied to or by objects in a class.
 Each operation has a target object as an implicit

argument. The behaviour of the operation
depends on the class of its target.
 The same operation may apply to many different

classes.

Amandeep S. Patti 7

Operations and Methods
 A method is the implementation of the operation
for a class.
 An operation may have arguments in addition to
its target object. Such arguments parameterize
the operation but do not affect the choice of
method.
 The method depends only on the class of the
target object.
 When an operation has methods on several
classes, it is important that the methods all have
the same signature.
Amandeep S. Patti 8

Notations for Class Diagram
 Place the name of the
Class Name
class in the first
Attribute: Type = Value partition (centered,
bolded, & capitalized),
list the attributes in the
second partition, and
Operation(arg_List): return type
write operations into
the third.

Amandeep S. Patti 9

Links and Associations
 Represent static relationship between
classes.
 A link is a physical or conceptual
connection between object instances.
 An association describes a group of links
with common structure and common
semantics.

Amandeep S. Patti 10

 All the links in an association connect
objects from the same classes.
 Both often appear as verbs in a problem
statement.
 Associations are inherently bi-directional.
 Associations are often implemented in
programming languages as pointers from
one object to another.


1 No more than one Company
0..1 Zero or One 1
* Many
0..* Zero or Many 1..*
1..* One or Many Person


Multiplicity
 It specifies how many instances of one
class may relate to a single instance of an
associated class.
 It constrains the number of related objects.
 It is generally described as being “one” or
“many”, but it is a subset of non-negative
integers.
 It depends on assumptions and how you
define the boundaries of a problem.

Class Name
Multiplicity Attribute: Type = Value


* {Constraints} *
For example, one
company will have one or
more employees, but 1..* 1
each employee works for
Class Name
one company only. Attribute: Type = Value



Link Attributes
 A link attribute is a property of the links in
an association.

Accessible by
File User

Access permission


Role Names
 A role is an end of an association.
 A binary association has two roles, each of which
may have a role name.
 A role is a name that uniquely identifies one end
of an association.
 Role names are necessary for associations
between two objects of same class.
Employee Employe
Person Works-for r Company


Ordering
 The ordering is an inherent part of the
association. An ordered set of objects on
the “many” end of an association is
indicated by writing “{ordered}” next to the
multiplicity dot for the role.
 This is a special kind of constraint.
{Ordered}
Window Screen
Visible-on


Qualification
 A qualified association relates two classes
and a qualifier.
 One to many and many to many
associations may be qualified.
 The qualifier is a special attribute that
reduces the effective multiplicity of an
association.


Qualification
 The qualifier distinguishes among the set
of objects at the many end of an
association.
 A qualified association can also be
considered as ternary association.
Directory File File
name


Aggregation
 Aggregation is the “part-whole” or “a-part-of”
relationship in which objects representing the
components of something are associated with
an object representing the entire assembly.
 The aggregate is semantically an extended
object that is treated as a unit in many
operations, although physically it is made of
several objects.


Aggregation
 Aggregation is inherently transitive; An
aggregate has parts, which may in turn
have parts.
 Many aggregate operations operate on
both direct and indirect parts.
 Aggregation can be fixed, variable or
recursive.


Types of Aggregation
 A fixed aggregate has a fixed structure; The
types and number of subparts are predefined.
 A variable aggregate has a finite number of
levels, but the number of parts may vary.
 A recursive aggregate contains directly or
indirectly, an instance of the same kind of
aggregate; the no. of potential levels is
unlimited.


Propagation of Operations
 Propagation is the automatic application of an
operation to a network of objects when the
operation is applied to some starting object.
 The propagation behaviour is bound to an
association, direction and operation.
 Propagation is indicated on object models with a
small arrow & operation name next to the
affected association. The arrow indicates the
direction of propagation.

Generalization
 It is the relationship between a class and
one or more refined versions of it. The
class being refined is called the superclass
and each refined version is called
subclass.
 It is sometimes called the “is-a” relationship
because each instance of a subclass is an
instance of superclass as well.

Generalization
 The notation for generalization is a triangle
connecting a superclass to its subclasses.
The superclass is connected by a line to
the apex of the triangle and the subclasses
are connected by the line to a horizontal
bar attached to the base of the triangle.


Generalization Figure
Color
Center position
 “Dimensionality” is Move
discriminator. select
 It is an attribute of
enumeration type that dimensionality

indicates which property
One Dim Two Dim
of an object is being
Orientation Orientation
abstracted by a
Fill type
particular generalization
Scale Scale
relationship.
fill


Generalization, Inheritance and
Specialization
 Generalization is used to refer to the
relationship among classes, while inheritance
refers to the mechanism of sharing attributes
and operations using the generalization
relationship.
 Generalization and specialization are two
different viewpoints of the same relationship,
viewed from superclass or from the subclass.


Aggregation V/s Generalization
 Aggregation relates instances. Two distinct
objects are involved; One of them is part of
other whereas generalization relates
classes and is a way of structuring the
description of a single object. Both super
class and subclass refer to properties of a
single object.


Aggregation V/s Generalization
 An aggregation tree is composed of object
instances that are all part of a composite
object; A generalization tree is composed
of classes that describe an object.
 Aggregation is often called “a-part-of”
relationship (sometime called “and-relationship”);
Generalization is often called “a-kind of” or
“is-a” relationship (sometime called “or-
relationship”).


Aggregation v/s Association
 Aggregation is a special form of association,
not an independent concept.
 Aggregation adds semantic connotations in
certain cases. If two objects are tightly
bound by a part-whole relationship, it is an
aggregation.
 If two objects are independent, and they are
linked, it is an association.


Abstract class
 An incomplete superclass that defines common
features to several classes.
 Not instantiated.
 Frequently used to define methods to be
inherited by subclasses.
 It can define the protocol for an operation
(abstract operation) without supplying a
corresponding method, for which each subclass
must provide its own implementation.


Concrete class
 Is a complete class.
 Describes a concept completely.
 Is intended to be instantiated.
 It may have abstract subclasses (but they in
turn must have concrete descendants).
 Only concrete classes may be leaf classes
in the inheritance tree.


Generalization as Extension and
Restriction
 In generalization, an instance of a class is an
instance of the class. Therefore all ancestor class
features must apply to the subclass instances. A
descendent class cannot omit or suppress an
ancestor attribute because then it could not truly be
an ancestor instance. Similarly operations on an
ancestor class must apply to all descendent classes.
A subclass may re-implement an operation for
reasons of efficiency but cannot change the external
protocol.
 Adding features is called Extension.
 Constraining ancestor attributes is called Restriction.

Overriding Operations
 The mechanism by which a child class can
provide an alternative implementation of a
method currently provided by a parent class.
 Overriding for extension: the new operation is the
same as the inherited operation, except it adds
some behaviour, usually affecting new attributes
of the subclass.
 Overriding for restriction: The new operation
restricts the protocol, such as tightening the
types of arguments.

Overriding Operations
 Overriding for optimization: An implementation
can take advantage of the constraints imposed
by a restriction to improve the code for an
operation, and it is valid use of overriding.
 Overriding for convenience: A common practice
in developing new classes is to look for a class
similar to what is desired. The new class is made
a subclass of the existing class and overrides the
methods that are inconvenient. This ad hoc use
of inheritance is semantically wrong.


Multiple Inheritance
 A class may inherits features from more than one
superclass. Such class is called a join class.
 A feature from the same ancestor class found
along more than one path is inherited only once.
 The advantage of multiple inheritance is greater
power in specifying classes and an increased
opportunity for reuse. It brings object modeling
closer to the way people thinks.
 The disadvantage is a loss of conceptual and
implementation simplicity.

Workarounds
Employee

Employee Employee
Payroll Pension

Hourly Salaried Exempt Vested Unvested
Employee Employee Employee Employee Employee

Multiple Inheritance Using delegation


Workarounds
Employee
Employee
Pension

Pay Status
Pension Status

Hourly Salaried Exempt Vested Unvested
Employee Employee Employee Employee Employee

Multiple Inheritance using inheritance & delegation


Metadata
 Data about data.
 Models are inherently metadata.

 Parts catalogs, blue-prints, dictionaries etc.

 Patterns and Metadata


Constraints
 Constraints are functional relationships between
entities of an object model. The term entity
includes objects, classes, attributes, links and
associations.
 A constraint restricts the values that entities can
assume.
 A good object model captures many constraints
through its structure.
 Object modeling notation for constraints is as follows: Constraints are
delimited by braces and positioned near the constrained entity. A
dotted line connects multiple constrained entities.


Constraints
Employe Window
e Length
Salary width
{Salary <= boss.Salary} {length > width}

Constraints on
objects

{ordered}
Country Office Person

Constraints on association
links

Derived Objects, Links and Attributes
 A derived object is defined as a function of
one or more objects, which in turn may be
derived.
 The notation for derived entity is a slash or
diagonal line (on the corner of a class box, or
an association line, or in front of an attribute).
Machine Assembly Part

offset offset Offset

Homomorphism
Maps Contains
Contains
CatalogItem Item
Model number Serial number
Describes

{Item1 contains item2 => item1.model contains item2.model}

Homomorphism for a parts
catalog


Object modeling

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