Object-Oriented Programming: Core Concepts and Java Implementation

Explains the core OOP concepts (encapsulation, inheritance, polymorphism), classes and instances, and an overview of UML diagram types.

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Object-Oriented Programming (OOP) is a programming paradigm that structures programs around “objects” as fundamental units. This improves code reusability, maintainability, and extensibility.

Components of an Object

Objects model real-world things and concepts, consisting of two elements:

  • Field: Represents the data and attributes (state) of an object. Defined as variables.
  • Method: Represents the behavior and operations of an object. Defined as functions.

Fields and methods together are called members.

Classes and Instances

  • Class: A blueprint or template for objects. Defines what fields and methods an object will have.
  • Instance: An actual object created in memory based on a class blueprint.

The operation of creating an instance from a class is called instantiation.

The Three Pillars of OOP

There are three particularly important concepts in object-oriented programming.

1. Encapsulation

A mechanism that hides an object’s internal structure (fields) from direct external access, allowing manipulation only through exposed methods.

  • Data hiding: Fields are declared with access modifiers like private to prevent direct external modification.
  • Exposing methods for field manipulation: public methods are provided for setting (setters) and getting (getters) field values.

This maintains object integrity, prevents unintended data modifications, and improves code maintainability.

2. Inheritance

A mechanism that allows a new class (subclass or child class) to inherit fields and methods from an existing class (superclass or parent class).

  • Improves code reusability.
  • Builds hierarchical structures between classes for systematic management.

3. Polymorphism

Meaning “many forms,” it refers to methods with the same name exhibiting different behavior depending on the object’s type (class).

  • Override: Redefining a parent class method in a child class.
  • Interface: A mechanism that enforces multiple classes to implement common methods.

Polymorphism improves code flexibility and enables uniform handling of different objects.

UML (Unified Modeling Language)

UML is a standardized notation for visually representing the design and structure of software systems. It is used in various phases of system development.

Common UML diagrams:

  • Class Diagram: Represents classes, their attributes, operations, and relationships (inheritance, associations, etc.) within a system.
  • Sequence Diagram: Represents the temporal sequence of messages (method calls) exchanged between objects.
  • Use Case Diagram: Represents the system’s functions (use cases) and the users (actors) who interact with them.
  • Activity Diagram: Represents the flow of a system or business process, including the sequence of activities and conditional branches.
  • Component Diagram: Represents the physical components (modules, libraries, etc.) that make up the system and their dependencies.

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