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Amandeep S. Patti 1
Object Modeling
OOADMCA 405 C (N2)
Amandeep S. Patti 2
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 3
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 4
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 5
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 6
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 7
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 8
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 9
Notations for Class Diagram Place the name of the
class in the first partition (centered, bolded, & capitalized), list the attributes in the second partition, and write operations into the third.
Class NameAttribute: Type = Value
Operation(arg_List): return type
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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.
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Links and Associations 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.
Amandeep S. Patti 12
Links and Associations
1 No more than one
0..1 Zero or One
* Many
0..* Zero or Many
1..* One or Many
Company
Person
1
1..*
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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.
Amandeep S. Patti 14
For example, one company will have one or more employees, but each employee works for
one company only.
Multiplicity
Operation(arg_List): return type
Attribute: Type = Value
Class Name
Operation(arg_List): return type
Attribute: Type = Value
Class Name
{Constraints}
**
1..* 1
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Link Attributes A link attribute is a property of the links in
an association.
Accessible by
Access permission
File User
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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.EmployeePerson Company
EmployerWorks-
for
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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.
Window Screen{Ordered} Visible-
on
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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.
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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 name
File
Amandeep S. Patti 20
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.
Amandeep S. Patti 21
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.
Amandeep S. Patti 22
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.
Amandeep S. Patti 23
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.
Amandeep S. Patti 24
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.
Amandeep S. Patti 25
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.
Amandeep S. Patti 26
Generalization “Dimensionality” is
discriminator. It is an attribute of
enumeration type that indicates which property of an object is being abstracted by a particular generalization relationship.
Figure
ColorCenter position
Moveselect
One Dim
Orientation
Scale
Two Dim
OrientationFill type
Scalefill
dimensionality
Amandeep S. Patti 27
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.
Amandeep S. Patti 28
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.
Amandeep S. Patti 29
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”).
Amandeep S. Patti 30
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.
Amandeep S. Patti 31
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.
Amandeep S. Patti 32
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.
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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.
Amandeep S. Patti 34
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.
Amandeep S. Patti 35
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.
Amandeep S. Patti 36
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.
Amandeep S. Patti 37
WorkaroundsEmployee
Employee
Payroll
Employee
Pension
Hourly
Employee
Salaried
Employee
Exempt
Employee
Vested
Employee
Unvested
EmployeeMultiple Inheritance Using delegation
Amandeep S. Patti 38
WorkaroundsEmployee
Employee
Pension
Hourly
Employee
Salaried
Employee
Exempt
Employee
Vested
Employee
Unvested
Employee
Pay Status
Pension Status
Multiple Inheritance using inheritance & delegation
Amandeep S. Patti 39
Metadata Data about data.
Models are inherently metadata. Parts catalogs, blue-prints, dictionaries etc.
Patterns and Metadata
Amandeep S. Patti 40
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.
Amandeep S. Patti 41
Constraints
Employee
Salary
{Salary <= boss.Salary}
WindowLength
width
{length > width}
Constraints on objects
Country Office Person{ordered}
Constraints on association links
Amandeep S. Patti 42
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
Offsetoffset
offset
Amandeep S. Patti 43
Homomorphism
CatalogItem
Item
Model number Serial numberDescribes
Maps ContainsContain
s
{Item1 contains item2 => item1.model contains item2.model}
Homomorphism for a parts catalog