Week 22 : Introduction to ObjectOriented Programming using PHP PART A

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Week 22 Introduction to Object-Oriented
Programmingusing PHPPART (A)
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What is O-O?

• Design, programming, and languages
• An alternative to procedural programming
• anything done using O-O can also be done using a
  procedural paradigm
• Considering a problem from the perspective of
  objects and how they interact

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Skills for O-O Programming

• Must have a solid understanding of subprograms
  and parameters
• stepwise refinement
• Should understand references/pointers
• Coding still involves basic concepts
• selection structures, repetition structures,

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O-O vs. Procedural

• Procedural Paradigm
• Program defines data and then calls subprogram to
  act on the data
• Object Paradigm
• Program creates objects that encapsulate the data
  and procedures that operate on the data

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Basic Concepts

• Objects
• Hidden data that is accessed and manipulated
  through a well-defined interface
• Classes
• A template or blueprint for creating objects,
  each with their own data
• Inheritance
• A new class created by modification of an
  existing class

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Objects

• Part of a program which
• models some real or conceptual object
• has behavioural responsibilities (behaviours)
• has informational responsibilities (attributes)

• Behaviours (methods)
• things an object can do
• like procedures and functions in other languages
• Attributes (fields)
• information an object knows (has-a)
• like data and variables in other languages
  (records)

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Classes

• A class is a template. No data is allocated until
  an object is created from the class. However
  attributes and behaviours are generically defined
• The creation (construction) of an object is
  called instantiation. The created object is often
  called an instance (or an instance of class X)

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Analogies for Classes Objects

• Class
• blueprint
• pattern
• cookie cutter
• factory

• Object
• building
• garment
• cookie
• widget

Platos allegory of the cave.
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Class Diagram
Internal/private/helper methods only available
within the class.
Defined by the class. Filled by the object.
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Encapsulation - Real Life

• Bank machine
• Hidden data
• account balance
• personal information
• Interface
• deposit, withdraw, transfer
• display account information

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Classes - Programming

• Karel the Robot
• Hidden data
• location
• number of Things
• Interface
• move, turnLeft
• pickThing, putThing

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Multiple Instances of a Class

• No limit to the number of objects that can be
  created from a class
• Each object is independent. Changing one object
  doesnt change the others

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Interaction Among Classes

• A program is composed of multiple classes
• Classes may contain references to other classes
  within the set of attributes or behaviours
• Start in an application class (main)
• construct one or more objects and call methods
  associated with those objects

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Using UML in Design

• every class has a box
• class name
• attributes/instance variables
• behaviours/methods
• Arrows indicate the relationships among classes
• indicates 0-many

Bank

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Inheritance

• The inheriting class contains all the attributes
  and behaviours of the class it inherited from
  plus any attributes and behaviours it defines
• The inheriting class can override the definition
  of existing methods by providing its own
  implementation
• The code of the inheriting class consists only of
  the changes and additions to the base class

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Inheritance Diagram
General Outline
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Why Use Inheritance?

• Modular coding
• less code, easier to understand
• Code reuse
• dont break what is already working
• easier updates
• May not have access to modify the original source
  code
• Polymorphism

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Inheritance Terminology

• Class one above
• Parent class, Super class
• Class one below
• Child class
• Class one or more above
• Ancestor class, Base class
• Class one or more below
• Descendent class

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Inheritance Bank Account
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Object-Oriented PHPPART (B)
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Structure of a class
class Classname var attribute1 function
operation1() function
Classname(param) echo "Constructor"

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Class structure in PHP 5
class Classname var attribute1 function
__construct(param) echo
"Constructor" function __destruct()

Constructor always named __construct
Will look for function with class name if no
__construct
Can define destructor (no parameters)
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Instantiating classes
class Classname function Classname(p)
echo "Constructor p\n" a new
Classname('First') b new Classname('Second')
Output
Constructor First Constructor Second
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Usingclassattributesandoperations
class Classname var foo function
bar(p) this-gtfoo p echo
this-gtfoo."\n" b new
Classname() b-gtbar('wow') echo b-gtfoo."\n"
Output
wow wow
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Implementing inheritance
class B extends A var attr2 function
op2() b new B() b-gtop1() //
defined in A
Attributes and operations defined in parent are
available in child class.
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Overriding

• Can override operations in parent
• No way to access original in PHP 4
• PHP 5 provides access with parent keyword
• parentop1()
• PHP 5 adds final keyword
• Operations declared as final cannot be overridden

final function op1()
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Multiple inheritance(actually the lack thereof)

• PHP does not allow multiple inheritance
• Can define interfaces in PHP 5
• Similar to interfaces in Java

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New OOP features in PHP 5

• __get and __set
• public, protected, private
• Per-class constants
• Static methods
• instanceof and type hinting
• Cloning and __clone
• Abstract classes
• __toString and other special functions

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__get and __set
class Classname var att1 function
__get(name) return this-gtname
function __set(name, value)
this-gtname value
Calls __set
What will the values of the parameters be?
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instanceof and type hinting
True if b is an object of class B
function check_hint(B obj) //
check_hint(a)
Error if a is not an object of class B
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PHP 5 vs. PHP 4Two notes

• Objects are passed by reference in PHP 5
• Passed by value in PHP 4
• Pass by reference is more efficient
• PHP 4 has "difficulty dereferencing objects that
  were returned from functions"
• e.g. select_object()-gtdisplay()
• This problem is fixed in PHP 5.