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Object-Oriented Principals Dwight Deugo Nesa Matic
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Transcript of Object-Oriented Principals Dwight Deugo Nesa Matic
Object-Oriented PrincipalsObject-Oriented PrincipalsDwight Deugo ([email protected])Dwight Deugo ([email protected])Nesa Matic ([email protected])Nesa Matic ([email protected])
www.espirity.com
2 © 2003-2004, Espirity Inc.
Additional Contributors None as of Septembe, 2004
3 © 2003-2004, Espirity Inc.
Module Overview1. Object-Oriented Principals
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Module Road Map1. Object-Oriented Principals
Objects Classes, instances, fields and methods Encapsulation Polymorphism Inheritance Dynamic binding
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What is Object-Oriented Technology? It is a technology that allows modeling
of software as the real world This is achieved through objects and
interactions between them It is a way of thinking of software as
reusable components Every object is a component on its own
and could be reused in different context
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Why Use Object-Oriented Systems? Benefits of Object-Oriented systems:
Greater flexibility Reusability of objects Higher quality control Lower maintenance
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Developing Object-Oriented Software Software is modeled by objects
Software objects represent real-world objects
Each object contains: State, i.e., data Behavior, i.e., methods/functions
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Object-Oriented Software Developing object-oriented software is
identifying: Objects Characteristics of individual objects Relationships between objects
Objects interact by sending messages to each other Interacting objects make an object-
oriented system
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Objects and Loose Coupling Changing an object’s data does not
lead to changes in an object’s external behavior An object’s external interface stays the
same Promotes loose coupling between objects
state
behavior
object1 state
behavior
object2
state
behavior
object3
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Objects Every object has:
State Behavior
State represents data - what an object knows, or what an object contains
Behavior represents what an object can do
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Person object State: age, name, children Behavior: addChild, getAge, setAge
Object State and Behavior
statebehavior
aPersonagenamechildren
getAgesetAgeaddChild…
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Interactions between Objects Object interact by sending messages to
each other Objects and interactions between them
make up an object-oriented systemaPolicyFactory:
createPolicy
aBroker: aPerson:
aPolicy:aPolicy
new(“Joe Smith”, “35 years”)aPerson
setOwner(aPerson)
…
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Messages There are two major terms in
messaging: Message sender Message receiver
Messages may have arguments
getAge()sender receiver
message
aBroker: aPerson:
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Methods Method is concrete implementation of
a message When message is sent to a receiver:
Method is found by type of the receiver object and method signature
Method code is executed Method represents an object’s
response to a message
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Method Signature Method signature is unique identifier of
the method It is used to distinguish methods with
same name and same number of parameters
It consists of: Method name Parameter name Parameter type
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Object’s Public Protocol Public protocol is set of messages that
can be sent to an object It does not include messages that an
object can send to itself These are private aPerson
agenamechildren
getAgesetAgeaddChild…
public protocol
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Fields Fields represent characteristics of an
object Fields are also known as attributes, or
instance variables aPersonagenamechildren
getAgesetAgeaddChild…
fields
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Object-Oriented Principal: Encapsulation
Objects hide implementation of the messages behind their public protocols Object’s internal implementation is
accessed by that object only Encapsulation is also known as
implementation hiding
getName()
aBroker: aPerson:
getAge()Joe Smith
35
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Classes Classes are:
Factories for creating objects Template for the same kind of
objects that describes their state and behavior
Code repository for objects Classes define objects (by
defining their state and behavior) and their type
PersonagenamechildrenaddressgetAgesetAgeaddChild…
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Instances Every object is an instance of some
class All instances of same class have the
same protocol They have same fields and same methods
that are defined by the classJohn Smith35John SmithJimmy, Chloe
Dad Smith60Dad SmithJohn, Tom
class
classPersonagenamechildrenaddressgetAgesetAgeaddChild…
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Object-Oriented Principal: Inheritance
Some classes may share commonalities For example HomePolicy, AutoPolicy, LifePolicy classes may all have same state and behavior
Instead of repeating commonalities in each class, we can abstract them in a common place These commonalities can be stored in a
super class Each subclass inherits state and behavior
from its superclass
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Specialization and GeneralizationPolicy
clientpremiumpolicyNumbergetClientsetClient…
HomePolicyhouse
getHousesetHouse
AutoPolicyauto
getAutosetAuto
LifePolicy
gene
raliz
atio
nspecialization
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Why Inheritance? Inheritance represents real-world
modeling Some objects are special cases of other
objects Inheritance promotes reuse and
extensibility Same data and behavior is shared among
objects of different types (different class) New data and new behavior that is
common for those objects is easier to add
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Object-Oriented Principal: Polymorphism
Polymorphism different objects respond to the same
message in different ways For example when asked to talk a dog barks,
and a cat meows It is often supported by method overriding
Overriding means that subclass may implement the same method as superclass, but with different code
toString() method in the Object class is an example of often overridden method
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Overriding Example Consider Animal class:
Dog and Cat as subclasses All Animal objects should know how to
talk
aCat.talk();
aDog.talk();
meowsbarks
Animaltalk()
Dogtalk()
Cattalk()
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Dynamic Binding Dynamic binding represents runtime
method binding It is runtime binding of method invoked in
the message to the method that implements that message
For example:anAnimal.talk();
aDog…
talk
aCat…
talk
27 © 2003-2004, Espirity Inc.
Labs!
Lab: Object-Oriented Concept