Introduction to Object Oriented Programming

Overview

• Procedural programming vs. OOP
• Classes
• Encapsulation
• Relationships between classes
• Objects and Instance variables
• Single responsibility
• Methods
• Object Construction
• Mutators & Accessors

What is procedural programming?

• Procedural (structured) programming consists of designing a set of procedures (algorithms) to solve a problem.
• The procedural paradigm suggests that a programmer will:
  • firstly, identify which algorithms to manipulate data
  • secondly, identify which structures use which algorithms.

Why do we use procedural programming?

• Small problems are easily resolved with a procedural implementation

Why do we not use procedural programming?

• Procedural implementations scale poorly.
• As the program grows in size, its complexity increases.
  • behaviors become tightly-coupled
  • debugging becomes more difficult
  • code-changes and maintenance becomes more difficult
  • testing a single aspect becomes nearly impossible

Why do we use OOP?

• Large problems can more be more easily scaled using the OOP paradigm.
• OOP allows users to break problem down into small logical objects
• OOP allows users to view code-details within the context of a specific object
• OOP allows users to more easily debug code
• OOP allows greater testability of code

Object Oriented Programming (OOP)

• An object-oriented program is made of objects
• Each object has specific functionalities, which users access via the object’s methods.
• The OOP paradigm suggests that a programmer will
  • firstly identify which structures to manipulate data
  • secondly identify what algorithms each structure will use

The 3 aspects of an object

• Identity - What is its location?
  • How is that object distinguished from other objects of the same type?
• State - What does it store?
  • What is the value of the internal objects this object contains?
• Behavior - How does it act?
  • What services or actions this object can perform?

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Classes

• A class is a template, or blueprint from which objects are made
  • it is the cookie-cutter to a cookie
  • it is the classification of an object.

Class naming conventions

• Class names must begin with a letter followed by any combination of letters, digits, and underscores.
  • By convention, class names start with a capital letter.
    • You cannot use a Java reserved word to name a variable or class.
• Whitespace is irrelevant to the Java compiler

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Encapulsation

• Classes encapsulate several data-fields and behaviors into a single entity.
• Encapsulation is combining class-members (methods and variables) in a single scope.

Encapsulation

• “Wraps” several data fields into a single entity

Sample

• Script

class Person(object): ## class signature
    ## constructor
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name

• Usage

def main():
    person = Person("Leon", "Hunter", 27)

if __name__ == '__main__':
    main()

• Output

Instance-Functions (Methods)

• behaviors of an object are made available to users via the object’s methods
• methods which are invoked on an object are instance-methods
• method-names should describe the intended behavior of the object
• methods of an object have hidden implementation
• methods describe a “can perform” relationship.

Instance-Variables (Fields)

• An instance-variable, or field are representative of the properties or attributes of a Class.
• By aggregating the values of an instance’s fields, we derive the instance’s state.
• field-values of an object are made available to users via the object’s getters
• variable-values of an object can be manipulated by the user via the object’s setters
• the aggregation of an object’s variable’s values determines the object’s state.
• fields describe a “has a” relationship.

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Constructors

• a method which creates a new instance of a class (an object)
• describes the initial state of an object

Nullary Constructor

• A method is said to be nullary when it takes no arguments as a parameter.

Sample

• Script

class Person(object): ## class signature

	def __init__(self): ## constructor signature
        print("A person object has been created")

• Usage

def main():
    person = Person()

if __name__ == '__main__':
    main()

• Output

A person object has been created

Default Constructor

• The default constructor is a nullary constructor with an empty body.
• If no constructor is defined explicitly in a class, the default constructor is provided by the compiler.

Sample

• Script

class Person(object): ## class signature
    ## no constructor defined

    def say_hello():
        print("Hello")

• Usage

def main():
    person = Person()

• Output

A person object has been created

Non-nullary Constructor

• A method is said to be non-nullary when it takes at least 1 argument as a parameter.

Sample

• Script

class Person(object): ## class signature
    ## constructor
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name

• Usage

def main():
    leon = Person("Leon", "Hunter", 27)


if __name__ == '__main__':
    main()

• Output

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Setters
(Mutators)

• Mutators change initial, or subsequent, values of an object.
• Setters are a specific type of mutator which expliticly reassign the reference of a field.

Sample

• Script

class Person(object): ## class signature
    ## constructor
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name

    def set_first_name(self, new_first_name):
        self.first_name = new_first_name

    def set_last_name(self, new_last_name):
        self.first_name = new_last_name

• Usage

def main():
    leon = Person("Leon", "Hunter", 27)
    bob_first_name = "Robert"
    leon.set_first_name(bob_first_name)


if __name__ == '__main__':
    main()

• Output

Getters
(Accessors)

• Accessors are methods used to retrieve or compute data from an object.
• Getters are specific types of accessors which retrieve a reference from the object.

Sample

• Script

class Person(object): ## class signature
    ## constructor
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name

    def set_first_name(self, new_first_name):
        self.first_name = new_first_name

    def set_last_name(self, new_last_name):
        self.first_name = new_last_name

• Usage

def main():
    leon = Person("Leon", "Hunter", 27)
    bob_first_name = "Robert"
    leon.set_first_name(bob_first_name)
    leon_new_first_name = leon.get_first_name()
    print(leon_new_first_name)


if __name__ == '__main__':
    main()

• Output

Robert