9. Object-Oriented design and UML (Unified Modeling Language)

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9. Object-Oriented design and UML (Unified
Modeling Language)

• In this part you will learn
• Definition of Object-Oriented design
• Encapsulation
• Information hiding
• Classes and Objects
• State retention
• Object identity
• Messages
• Inheritance
• Association
• Polymorphism
• Generosity
• All of these concepts are represented with UML
  notation.

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8.1 Definition of Object-Oriented design

• Before giving a definition, lets first
  understand the difference in the principles of
  structured design and object-oriented design.
• Structured design function-oriented or
• model-oriented.
• For example, a cash dispenser is modeled as
  follows

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• Object-oriented design object-oriented.
• Example,

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• It is difficult to give a definition for
  Object-Oriented design
• mathematically, but we can describe what
  object-orientation
• is about in an informal manner.
• Description object-oriented design (OOD) means
  to build programs by means of designing objects
  and their relations.
• In other words, in OOD identification and design
  of objects and their relations are the most
  important tasks. If all of the necessary objects
  and their relations are well designed, the entire
  program will be well designed.

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What is an object in general

• Description an object is a material thing that
  can be seen or touched.
• Examples
• A watch a place to show time operations (e.g.,
  (1) adjust time, (2) set alarm, (3) adjust dates)
• A TV set a screen operations (e.g., (1) change
  channels, (2) adjust volume, (3) Change modes)
• A book a set of pages, but no operations to
  offer.
• A person physical body operations (e.g., (1)
  eat, (2) walk, (3) drink, (4) sleep)

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8.2 Encapsulation

• Object-oriented encapsulation is the packaging of
  operations and attributes representing state into
  an object type so that state is accessible or
  modifiable only via the interface provided by the
  encapsulation.
• Attributes are represented by instance variables
  in a Java class.
• Operations are like methods of a Java class.
• State is a collection of instance variables
  together with their values.
• Interface are like public methods of a Java
  class.

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The illustration of encapsulation

• class Calculator
• private int reg
• public Calculator()
• initialize reg
• //constructor of the class
• public int Add(int i)
• reg reg i
• public int Subtract(int i)
• reg reg i
• private int Multiply(int i)
• reg reg i

Attribute reg Operations Add,
Subtract,
Multiply State reg Interface Add,
Subtract
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The change of interface

• class Calculator
• private int reg
• public Calculator()
• initialize reg
• //constructor of the class
• public int Add(int i)
• reg reg i
• public int Subtract(int i)
• reg reg i
• public int Multiply(int i)
• reg reg i

Attribute reg Operations Add,
Subtract,
Multiply State reg Interface Add,
Subtract, Multiply
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8.3 Information hiding

• Information hiding is the use of encapsulation to
  
• restrict from external visibility certain
  information
• or implementation decisions that are internal to
  the
• encapsulation structure.
• Information means
• Attributes. (1) the implementation details of
  attributes, including how they are stored and
  named, are hidden, (2) they are usually not
  directly accessible by outside operations.
• Implementation details of all the operations.
• For example

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• class Calculator
• private int reg //hidden
  attribute
• public Calculator()
• initialize reg
• //constructor of the class
• public int Add(int i) //interface
  operation
• reg reg i
• public int Subtract(int i) //interface
  operation
• reg reg i
• public int Multiply(int i) //interface
  operation
• reg reg i

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8.4 Classes and objects

• Description(class) a class can be understood in
  both static and dynamic views
• Static view a class is a specification that
  defines the attributes and operations which its
  every object shares. Therefore, a class is what
  you design and program.
• Dynamic view a class is a collection of objects
  that share the same features (i.e., with the same
  kinds of attributes and operations). Therefore,
  objects are what you create (from a class) at
  run-time.

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• Description(object) an object is an instance of
  a class. In other words, an object can be
  instantiated from a class.
• Graphical illustration of class and objects x1,
  x2, and x3 represent objects of the class that
  has three operations Op1, Op2, and Op3.

Op1
x1
class
Op2
x2
Op3
x3
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Graphical representations of classes and objects
in UML

• UML Unified Modeling Language.
• Initially, UML was proposed by Bouch, Jacobson,
• and Rumbaugh, and then standardized by
• Object Management Group (OMG).

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or
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Example of class
/ age means the attribute is not directly
settable, it is read-only variable. Keywords in,
out, inout. in argument input of the method. in
is usually omitted. out argument output of the
method. inout argument both input and output of
the method.
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Example of object
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• Suppose that three objects are instantiated from
  class
• Person
• object1 object2
  object3

person1
person2
person3
var name var dateOfBirth var height var age
var name var dateOfBirth var height var age
var name var dateOfBirth var height var age
setName getName getHeight setHeight
setName getName getHeight setHeight
setName getName getHeight setHeight
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• Since the methods are program codes, and can be
• shared by all of the objects instantiated from
  the
• same class, the implementation of objects of
  class
• Person at run time is in fact as follows
• methods object1
  object2

var name var dateOfBirth var height var age
var name var dateOfBirth var height var age
setName getName getHeight setHeight
var name var dateOfBirth var height var age
object3
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8.5 State retention

• Definition (state) state of an object is a set
  of attributes, which is subject to change during
  the operation of the program.
• For example, the object realPerson has a state
• name Mike
• dateOfBirth 25.2.1965
• height 175
• age 20

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• Description state retention means that the state
  of an object is retained after it has finished
  executing its methods. In other words, an object
  sustains its state until it dies (e.g., garbage
  collection).
• This point is different from a function in C and
  a procedure in Pascal when calling a function or
  procedure, the state of the function or procedure
  is created, and when the function or procedure
  terminates, its state vanishes.

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8.6 Object identity

• Object identity is the property by which each
  object
• (regardless of its class or current state) can be
  
• identified and treated as a distinct software
  entity.
• In other words, every object has a unique
  identity.
• Such an object identity is called object
  reference
• or object handle. Such a handle is decided when
• an object is created with the new operator. It is
  a common
• way to use the address of the object (the
  starting memory
• unit of its attributes) as its handle.

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Example

• Person p1 new Person()
• Person p2 new Person()

602237
P1
objects
142857
P2
p2 p1
object
602237
P1
This object has died.
602237
P2
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8.7 Messages

• Description A message is the vehicle by which a
  sender object obj1 conveys to a target object
  obj2 a demand for object obj2 to apply one of its
  methods.

obj2.m1(in a, out b)
obj2
obj1
Sending a message is like calling a
traditional function or procedure
call m1(obj2, a, b)
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8.7.1 Message structure

• The general structure of a message is
• obj2.m1(in a, b, c, out x, y, z)
• method name
  output arguments
• target object input
  arguments
• 
  message

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8.7.2 The roles of objects in messages

• An object may play the following roles in
  messages
• The sender of a message (e.g., obj1).
• The target of a message (e.g., obj2).
• Pointed to by an argument passed back or forth in
  a message (e.g., obj).
• obj2.m1(in a, b, c, out x, y, z,
  inout obj)

obj1
obj2
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8.7.3 Types of messages

• There are three types of messages
• Informative messages
• Interrogative messages
• Imperative messages

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• Informative messages
• An informative message is a message to an
  object that provides the object with information
  to update itself. It is also known as update,
  forward, or push message.
• For example,
• person1.setName(name1)
• In this message object person1 is informed that
  its name is changed to name1. Therefore, person1
  needs to update its name using the method setName.

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• Interrogative message
• An interrogative message is a message to an
  object requesting it to reveal some information
  about itself. (It is also known as a read,
  backward, or pull message.).
• For example,
• person1.getName()
• Object person1 is requested to tell its name
  by calling its getName method.

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• Imperative message
• An imperative message is a message to an
  object that requests the object to take some
  action on itself, another object, or even the
  environment around the system. (It is also known
  as a force or action message.)
• For example,
• person1.sendName(obj)
• This message requests object person1 to send
  its name to another object obj of class Person.

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8.8 Inheritance

• Question
• In your design, if you wrote a class C and then
  later
• discovered a class D that was almost identical to
  C
• except for a few extra attributes or operations,
  what
• would you do?
• Example class C class
  D
• T1 a1
  a1, a2, method1, and
• T2 a2
  method2 of C are
• method1
  needed here
• method2
  method3
• 
  

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• Two solutions
• Duplicate all the attributes and operations of C
  and put them into D.

class C class D T1 a1
T1 a1 T2 a2
T2 a2 method1
method1 method2
method2
method3

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• Have class D use the attributes and operations of
  the class C. This solution is called inheritance.
• Description Inheritance (by class D from C) is a
  mechanism that allows a class D to have
  implicitly the attributes and operations of class
  C, as if those attributes and operations had been
  defined upon D itself.
• In other words, through inheritance, objects of
  class D can make use of attributes and operations
  that would otherwise be available only to objects
  of class C.

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• Inheritance represents another major way in which
  
• object orientation departs from traditional
  systems
• approaches. It effectively allows you to build
• software incrementally in this way
• First, build classes to cope with the most
  general case.
• Then, in order to deal with special cases, add
  more specialized classes that inherit from the
  first class. These new classes will be entitled
  to use all the operations and attributes (both
  class and instance operations and attributes) of
  the original class.

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• For example,

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• Definition if class B inherits from class A, B
  is known as a subclass of A, and A is a super
  class of B.
• Inheritance is transitive. That is, if C
  inherits from B, and B inherits from A, then C
  will definitely inherit from A.
• In UML, the inheritance hierarchy
• classes B, C, and D inherit from A,
• class E inherits from C,
• are represented by the following graphical
  notation.

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Class inheritance hierarchy
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Multiple inheritance

• Description multiple inheritance is a mechanism
  that allows a class to inherit from more than one
  super classes. For example, a Professor can be an
  Employee and an Employer simultaneously.

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The problem in multiple inheritance

• The problem is the conflict of attribute or
  operation names in super classes.

Both super classes have the attribute name, and
the method getName.
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Solutions for the problem in multiple inheritance

• Resolve the conflict of attribute and operation
  names in the super classes.
• Run time error
• Check by the complier

• Disallow the multiple inheritance (e.g., Java)

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Exercise 7

• Describe the relationship between objects and
  classes.
• (2) Explain what is object-oriented design.
• (3) Suppose Animal be the super class of classes
  Dog, Cow, and Cat Cat is the super class of
  classes WhiteCat and BlackCat. Draw the class
  inheritance hierarchy of these classes.

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8.9 Association

• Description an association (known as binary
  association) represents a relation between two
  classes, where a relation is a set of
  relationships between instances of the classes.
• Example let LibraryPatron and LibraryBook be
  two
• classes.
• An association between them is Borrowing, a set
  of relationship links stating which patron is
  currently borrowing which book. As an example,
  the Borrowing association may contain the
  following four links

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• Jeff is borrowing Program Design
• Chris is borrowing Programming in Java
• John is borrowing Software Engineering
• Jim is borrowing Formal Methods
• Notice that borrowing represents a bunch of
  links, each reflecting a relationship between two
  instances of two classes.

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8.9.1 The basic UML notation for associations

• The following figure shows three associations
  Employment between classes Person and Company
  Residence between Person and County and Site
  between County and Company.

Company
employee
0..1
Person
Employment
0..
employer
resident
0..
0..
Residence
Site
1..
1..1
County
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• The meaning of each component of this diagram
• The box represents a class.
• The line between two classes represents a
  relation (association) between the two classes. A
  relation has a name attached to the line. The
  name of a relation usually starts with a capital
  letter, like Employment and Residence.
• The role of a class in the association diagram
  may appear beside it at the end of a association
  line, such as employee and residence.
• The multiplicity of the association appears at
  the ends of each line. For example, 0..1 at the
  end of Employment association, beside class
  Company, means that a given person is an employee
  of 0 or 1 company, while 0.. at the end of the
  same association line beside class Person means
  that a given company is an employer of 0 to many
  (denoted by an asterisk ) persons.

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• Some important points about UML notation for
• associations
• The name of an association should be a noun,
  because it represents a class in implementation,
  as we will explain later.
• UML doesnt insist on a name for an association,
  but it is a good discipline to give a name for
  each association in general.
• UML doesnt require role names of classes either.
  The role names should be given whenever it is
  necessary in avoiding the confusion of the
  meaning of associations.
• The multiplicity can be abbreviated. For example,
  0.. can be written as , and 1..1 can be written
  as 1. But notice that 0..1 cannot be written as 1.

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8.9.2 Associations depicted as classes

• An association between two classes can be
  depicted
• as a class. This also indicates a way to
  implement an
• association in the program code.

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• Employment is depicted as a class in which Person
  and Company
• may be used to declare instance variables
  (representing attributes of
• its objects), like employee, employer, startDay,
  and terminationDay.
• Operations may be defined in this class for
  providing
• necessary services, such as setStartDay() and
• GetTerminationDay().

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8.9.3 Higher-order associations

• A higher-order association is an association
  among
• more than two classes.
• A diamond is used to represent a higher-order
• association.
• Example, the following Figure shows part of a
• purchasing model for buying items from vendors.
  In this
• business, the unit price depends on three
  factors the item type
• (the product), the company that is selling it
  (the vendor), and the
• quantity of items you purchase (the price-break
  level).

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• We build a three-way association among ItemType,
  Company, and
• PurchasedQuantity, in order to have a suitable
  home for the unitPrice
• attribute, which is determined by the instances
  of those three classes.

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• The operation getPrice() may be implemented
  simply in
• Java as follows
• public void getPrice(ItemType item,
• Company
  vendor,
• 
  PurchasedQuantity quantity)
• if (item.getType() Pen
• vendor.getName() NEC
• quantity.getNumber() gt 20)
• unitPrice Exp1
• else
• unitPrice Exp2

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8.9.4 Navigability of associations

• Navigability of an association is the ability of
• showing the direction of the association between
  the
• two classes.
• Example 1

This diagram shows that classes Person and
Company have the association Employment, and
associated Company object can be easily and
quickly (supported by implementation) found from
a Person object.
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Such a navigability can be built into each Person
object by referring to a Company object, as shown
in the following diagram.
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• Example 2

This diagram shows that classes Person and
Company have the association Employment, and
associated Person object can be easily and
quickly (supported by implementation) found from
a Company object.
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Such a navigability can be built into each
Company object by referring to a set of Person
objects, as shown in the following diagram.
Where Set ltPersongt means a set of Person objects.
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• Example 3

This diagram shows that classes Person and
Company have the association Employment, and
associated Person object can be easily and
quickly (supported by implementation) found from
a Company object, and vice versa.
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Such a navigability can be built into both a
Company object by referring to a set of Person
objects and a Person object by referring to a
Company object, as shown in the following diagram.
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8.10. Whole/Part associations

• The whole/part association is an association
  between classes.
• There are two kinds of whole/part associations
• composition and aggregation.

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8.10.1 Composition

• Description if an object A is comprised of
  objects A1, A2, and A3, we say A is the
  composition of A1, A2, and A3.
• Examples
• A dog is a composition of a head, a body, a tail,
  and four legs.
• A university is a composition of departments,
  professors, students, administration officers,
  buildings, and equipments.
• A shop is a composition of goods, shop
  assistants, and a building.

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• Usually, in a composition the whole is called
  the composite object while the part is called
  the component object.
• Example
• A is the composite object while A1 (the same
  as for A2 and A3) is the component object. Such a
  composition association is depicted in UML by the
  following diagram.

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• Such a composition can be implemented by
• declaring three instance variables in A-Class
• A1 A1-Class
• A2 A2-Class
• A3 A3-Class
• Important characteristics of composition
• The composite object does not exist without its
  components. For example, a dog does not exist
  without a head.
• (2) The component objects of a composite object
  may belong to different classes. For example, a
  head, a body, a leg, a tail of a dog usually
  belong to different classes.

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8.10.2 Aggregation

• Description aggregation is a group/member
  association.
• Examples
• A street is an aggregate of houses.
• (2) A forest is an aggregate of trees.
• (3) A club is an aggregate of club members.
• (4) A book is an aggregate of chapters.
• In the aggregation association, the whole is
  called the aggregate object while the part is
  called the constituent object.

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• Important characteristics of aggregation
• The aggregate object may potentially exist
  without its constituent objects (which is
  different from a composite object in a
  composition). For example, a street may still
  exist even if all of its houses are destroyed.
• (2) The constituent objects of a typical
  aggregate object belong to the same class (which
  is different from a component object in a
  composition). For example, an aggregate object
  forest consists of trees that belong to the same
  class.

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• The UML representation of aggregation

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• These two aggregations can be implemented by
• declaring an instance variable in each of classes
• Book and Club.
• In class Book
• chapters Set ltChaptergt
• In class Club
• members Set ltPersongt

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8.11 Polymorphism

• The word polymorphism comes from two Greek
• words that mean, respectively, many and form.
  
• Polymorphism is an important feature of object-
• oriented programs. It is realized due to the
  class
• inheritance.

Description polymorphism is the facility by
which a single operation or attribute name may be
defined upon more than one class and may take on
different implementations in each of those
classes.
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• Example

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• In this example, Polygon includes three specific
• shapes rectangle, triangle, and diamond.
• We define each shape as a class, and build the
  class
• hierarchy on the previous page. In this hierarchy
  
• Polygon is the super class, which has an
  attribute
• area and an operation getArea().
• Classes rectangle, triangle, and diamond are all
• subclasses of Polygon. They all inherit Polygons
  attributes and operation, including getArea().
  However, the implementation of the getArea() on
  each subclass may be different because the
  formula for computing the area of each specific
  shape may be different. The

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• Formulas for computing areas for different
  shapes
• Rectangle
• area length width
• (2) Triangle
• area (base height) / 2
• (3) Diamond
• area (diagonal-line1 diagonal-line2)
  / 2

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• Description overriding is the redefinition of an
  operation
• defined on a class C in one of its subclasses.
• For example,
• getArea() defined in class Rectangle is an
  overriding of
• getArea() defined in class Polygon.
• getArea() defined in class Triangle is also an
  overriding of
• getArea() defined in class Polygon.
• getArea() defined in class Diamond is also an
  overriding of
• getArea() defined in class Polygon.

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• The explanation of polymorphism an example of
  Java method

void A() Polygon p new Polygon()
Triangle t new Triangle() Diamond d new
Diamond() if (e) p t else p d
p.getArea()

• p.getArea() has the following possbilities
• p is bound to an object of Triangle, and
  getArea() defined in Triangle is executed.
• p is bound to an object of Diamond, and getArea()
  defined in Diamond is executed.
• p is bound to an object of Rectangle, and
  getArea() defined in Retangle is executed.
• p is bound to an object of Polygon, and getArea()
  defined in Polygon is executed.

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• Dynamic binding (or run-time binding) is the
  technique by
• which the exact piece of code to be executed is
  determined
• only at run-time (as opposed to compile-time).
  The principle
• of such a binding is to execute the operation of
  the current
• object, if it is defined in the corresponding
  class of this
• object, then, if not, to execute the same
  operation defined in
• its super class.

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8.12 Genericity
A simple example of class class A Triangle
v1 int v2 void M1(Triangle w)

class A T v1 int v2 void M1(T w)

More general class
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• Genericity is such a construction of a class C
  that
• one or more of the classes that it uses
  internally is
• supplied only at run-time (at the time that an
  object
• of class C is instantiated).
• Such a class C is usually called parameterized
  class (also
• known as generic class). A parameterized class is
  
• represented graphically in UML as follows

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• Declare variables with a parameterized class
• Example
• s Stack ltintgt
• This declaration means that variable s will be
  used as a stack of integers.
• Define a specific class with a parameterized
  class in UML

Then the class IntStack can be used to declare
variables s IntStack
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Exercise 8

• Suppose the classes Student and University have a
  binary association Study, draw a UML association
  diagram to describe this association. In this
  diagram you must show appropriate multiplicity.
• (2) Let class A be the composition of classes B,
  C, and D. Draw an appropriate UML diagram to
  describe this composition.