Object-Oriented Programming Principles

• This lecture will enable students to:
  • Differentiate between Procedural and Object-Oriented Programming.
  • Explain Encapsulation and Information Hiding concepts and their purpose.
  • Define and Differentiate between Classes and Objects.
  • Utilize Classes to model objects.
  • Discern between public and private access modifiers and their purpose.
  • Construct accessor and mutators.
  • Access object's data and methods using object member access operator.
  • Distinguish between instance and static variables and methods.
  • Draw UML diagram to describe classes and objects.
  • Define the shadowing problem and explain how to avoid it.
  • Apply Object-Oriented design in solving problems.

Procedural Programming

• Code is organized around procedures or functions.
• Functions manipulate data, which is typically stored in data structures like arrays and structs.
• Focus is on the procedures or functions that perform operations on the data.
• There is no concept of classes or objects.

Object-Oriented Programming (OOP)

• OOP revolves around objects that encapsulate data and behavior.
• Classes act as templates for creating objects, defining properties (attributes) and behaviors (methods).
• Objects are instances of classes, representing real-world entities with states (data) and actions (methods).

Fields and Methods

• Fields define the state of an object.
  • Examples for an alarm clock: current second (0-59), current minute (0-59), current hour (1-12), alarm set time, and alarm on/off status.
• Methods are used to change a field’s value.
  • Includes setting the time, setting the alarm, turning the alarm on/off, incrementing the current second/minute/hour, and sounding the alarm.

Public vs Private Methods

• Public methods are accessed by users external to the object.
• Private methods form part of the object's internal implementation.

Class versus Object

• Class is a collection of similar objects, is conceptual and doesnt use memory
• Objects are instances of a class is real and uses memory
• Class consists of variables and methods
• Object consists of attributes and behaviours

Components of a Class

• Variables
• Behaviours
  • Constructors
  • Mutators
  • Accessors
  • User-defined methods

Access Modifiers

• Access modifiers indicates how a field or method can be accessed.

Class Variables Declaration

• Determine the Access Modifiers:
  • public
  • private
  • protected
  • default

Constructors

• Initialize data members of an object when a new object of the class is created.
• Same name as the class
• Must be declared public.
• Does not have any return type, not even void.
• Types
  • Default constructor: no-arg constructor.
    • Can define own no-arg constructor
  • Overloading constructor
    • Same name (name of the class) with different number of parameters/arguments

This Keyword

• A reference variable in Java that refers to the current object.
• Used to:
  • Refer to instance variables of the current class.
  • Invoke or initiate the current class constructor.
  • Can be passed as an argument in the method call or in the constructor call.
  • Return the current class instance.

Accessors (Getters)

• Used to get and return the value of a private field.

Mutators (Setters)

• Used to set the value of a private field.

UML

• UML (Unified Modeling Language) diagrams visually represent classes, their attributes, and methods.

Instance Fields and Methods

• Each class instance has a distinct set of fields (Instance Fields).
• Fields/methods declared previously are instance fields/methods.
• Objects from a class each have copies of instance fields.
• Instance methods are methods not declared with the static keyword.
• Need an object to be created to use Instance fields/methods.

Shadowing

• Occurs when a method has a local variable with the same name as an instance field.
• The local variable hides instance field value.
• Not recommended and local/instance variable names shouldn't match to avoid elusive bugs.