Transcript Classes and Objects

Chapter 1
Introduction to Object-Oriented
Programming and Software
Development
Chapter 1 - 1
Objectives
•Understand the basic of object-oriented
programming
– Differentiate classes and objects.
– Differentiate class and instance methods.
– Differentiate class and instance data values.
– Draw UML diagrams for classes and objects
– Describe significance of inheritance in object-oriented
programs
•Name and explain the stages of the software
lifecycle
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What is an Object ?
• Real-world objects:
– Concrete objects: Apple1, Car1, TV2, Teacher2,
Student3, …
– Conceptual Objects: 1, 2.3, Date1, Meeting2, point2, …
• Objects have:
– Properties (attributes): color, weight, height, sex, name,
speed, position,…
– Capabilities (behaviors): can receive commands(,
request, query) and respond (do actions) based on its
internal states to change its internal state and/or
external environment.
• The properties of an object constitutes its current
state.
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What is a Software object ?
• a software bundle of data and functions used to
model real-world objects you find in everyday life.
• In OOP, Software objects are building block of software
systems
– program is a collection of interacting objects
– objects cooperate to complete a task
– to do this, they communicate by sending “messages” to each other
• Software objects can model
– tangible things: School, Car, Bicycle,
– conceptual things: meeting, date
– Processes: finding paths, sorting cards
• Note: Software objects are only abstraction of real-world
objects; properties and behavior of irrelevance will not be
modeled in software objects.
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What is a Java Object?
• In Java, an object consists of 0 or more fields and 0 or more
methods.
– Fields are used to model properties.
– Methods are used to model capabilities.
• Fields are variables.
– like the fields of a C struct.
– An object without methods is equivalent to a C struct.
• A method is similar to a C function.
– Normally, it will operate on the fields of the object.
– These fields are accessible by name in the method.
• Java variables can not hold objects, but only references to
them.
– Object do not have a names.
– Object are created only at runtime.
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Classes and Objects
• Current conception:
– a java/software object  a real-life object,
– e.g., a Java car  a real car
• Disadvantage: impractical to work with objects this way
– may be indefinitely many (i.e., modeling all atoms in the universe)
– do not want to describe each individual separately, because they
may have much in common
• Classifying objects into classes of similar
properties/behaviors
–
–
–
–
–
factors out commonality among sets of similar objects
lets us describe what is common once
then “stamp out” any number of copies later
Ex: Student: { S1, S2, S3 } Course:{C1,C2,C3} Teacher:{ T1,T2}
but not {s1, t1}, {s2, t2}, {c1,c2,c3,s3}
• Analog:
– blueprint
– constructions
(class)
(objects)
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What is a Java Class?
• In Java, a class is a template (textual
description) of a set of similar objects.
– All objects in the class have the same types of
properties and the same set of capabilities.
• It defines the fields and methods that all
objects in that class will have.
– Classes have names.
– Class appear in the text of your program.
– A java program consists of a set of classes.
• A defined class is a Java Type, so you can
have objects or variables of that type.
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Example
class Person {
// fields or properties
int age ;
String name ;
Person father, mother ;
Person[] : siblings
// methods or behavior capability
void eat( Food food) ;
void play() ;
void work() ;
void sleep(); }
//possible statements
m1 = new Person(); p1 = new Person;
p1.mother = m1;
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Classes and Objects
• In OOP, a running application is results of
participating objects and their interactions.
• Similar objects are described or defined by the
same (set of ) classes
– properties  fields
– behavior capability  methods
• An object is called an instance of its defined
classes.
– Ex: stusent1 is an instance of both class Student and
Person and Object(萬物).
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Graphical Representation of a Class
<Class Name>
Example:
Account
We use a rectangle to
represent a class with
its name appearing
inside the rectangle.
Motorcycle
The notation we used here is based on the industry standard notation
called UML, which stands for Unified Modeling Language.
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Graphical Representation of an Object
<Object Name>
We use a rectangle to
represent an object and
place the underlined
name of the object
inside the rectangle.
Example:
SV198
This is an object named
SV198.
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An Object with the Class Name
<Object Name> : <Class Name>
This notation indicates
the class which the
object is an instance.
Example:
SV198 : BankAccount
This tells an object
SV198 is an instance of
the BankAccount class.
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Messages and Methods
• How do objects interact ?
– message passing ; method requests/invocation/call
• To instruct a class or an object to perform a task, we
– send a message (method call) to it.
– the message must be understandable to the receiving classes
or objects. i.e.,
– the receiving class or an object must possess a matching
method to be able to handle the received message.
• Kinds of method:
– A method defined for a class is called a class method, and
– a method defined for all instances of a class is called an
instance method.
• A associated value we pass to an object when sending
a message is called an argument of the message.
– e.g., p1.eat( apple1 ); // receiver.method ( argument …).
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Sending a Message
Message deposit
with the argument
250.00 is sent to a
BankAccount object
SV198.
caller
Rreceiver
deposit 250.00
SV198 : BankAccount
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Sending a Message and Getting an Answer
Ask for the current
balance of this
particular account.
getCurrentBalance()
SV198 : BankAccount
current balance
The current balance of
SV198 is returned.
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Calling a Class Method
Ask for the maximum
possible speed for all
MobileRobot objects is
returned.
MobileRobot
getMaximumSpeed()
maximum speed
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Class and Instance Data Values
• An object is composed of data values (for its properties)
and methods.
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–
–
–
Property has name, type and value
ex: int age = 10 ;
property name and type are static (time-invariant )
property value may change with time.
• An instance data value is a value of one of its properties of
an individual instance.
– For example, each BankAccount object maintains its balance.
• A class data value is a value of some property of the whole
class.
– It is shared by all instances of the class.
– Ex: minimum balance and average balance in Account class.
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Sample Instance Data Value
SV129 : BankAccount
SV098 : BankAccount
SV211 : BankAccount
current balance
908.55
current balance
1304.98
current balance
All three BankAccount
objects possess the
same current balance
property.
354.00
The actual dollar
amounts (data value)
are, of course,
different.
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Sample Class Data Value
BankAccount
minimum balance
100.00
There is one copy of
minimum balance for
the whole class and
shared by all instances.
This line is an
instance-of
relationship.
SV129 : BankAccount
SV098 : BankAccount
SV211 : BankAccount
current balance
908.55
current balance
1304.98
current balance
354.00
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Object Icon with Class Data Value
SV129 : BankAccount
minimum balance
100.00
current balance
908.55
When the class icon is
not shown, we include
the class data value in
the object icon itself.
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Class and object diagrams
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Inheritance
• Inheritance is a mechanism in OOP to design two
or more entities that are different but share many
common features.
– design/programming by difference.
– Features common to all classes are defined in the
superclass.
– The classes that inherit common features from the
superclass are called subclasses.
• We also call the superclass an ancestor and the subclass a
descendant.
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A Sample Inheritance
• Here are the superclass Account and its
subclasses Savings and Checking.
Account
Checking
Savings
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Inheritance Hierarchy
• An example of inheritance hierarchy among
different types of students.
Student
Graduate
Masters
Doctoral
Undergrad
Law
Commuting
Resident
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Software Engineering
• Much like building a skyscraper, we need a
disciplined approach in developing complex
software applications.
• Software engineering is the application of a
systematic and disciplined approach to the
development, testing, and maintenance of a
program.
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Software Life Cycle
• The sequence of stages from conception to
operation of a program is called software life
cycle.
• Five stages are
–
–
–
–
–
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Analysis
Design
Coding
Testing
Operation and Maintenance
Update and Evolution
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