The Pillars of Object-Oriented Programming

The purpose of this repository is to categorize my notes accordingly to the pillars of OOP. Additionally, the Java projects above all utilize OOP concepts and best practises in one way or another, while some projects may also show poor practise as well.

Table of Contents

1. General Knowledge
2. Encapsulation
• Data Hiding
• Information Hiding
• Immutability
• Access Modifiers
• Defensive Copying
3. Inheritance
4. Abstraction
5. Polymorphism
• Dynamic Dispatching
• Dynamic Binding
• Method Overriding
6. Object Relationships
• Aggregation
• Composition
• Collections as Fields
• Shallow Copy
• Deep Copy
• Defensive Copy
7. Other Key Concepts / Best Practises
• Enums (Enumerated Types)
• Class Invariants
• JUnit

General Knowledge

Encapsulation

Inheritance

Abstraction

• What is the difference between Abstract Class and an Interface?

Interface overview

The main structural element in Java that enforces an API is an interface

An interface contains only constants and abstract methods (with addition to default methods and static methods). The abstract methods are public by default.

It has no constructors and can't directly be instantiated

A class that implements an interface must implement aLL of the methods declared in the interface (no inheritance); otherwise won't compile

A class can have multiple inheritance

Abstract Class overview

A class extends to an abstract class. (E.g., a Dog IS-A Animal)

Methods: Abstract classes can have both abstract methods and concrete methods. Subclasses can inherit these concrete methods as-is or override them, and obviously, subclasses are required to provide their own implementation of abstract methods.

Constructors: Abstract classes can have constructors, and when a subclass is instantiated, the constructor of the abstract class is called before the constructor of the subclass.

Fields: There are no limitations as to what type of field you would like to declare or initialize in an abstract class.

Inheritance (IS-A): Single inheritance; A class can only extend one abstract class. (E.g., a Dog IS-A Animal)

Instantiation You cannot create an instance of an abstract class directly using the new keyword. You can only create instances of concrete subclasses of the abstract class.

Polymorphism

Says variable husky is declared as Dog Type, but is instanceof and references an object of Husky Type.

Collections as Fields: Shallow Copy vs. Deep Copy

Object Relationships: Aggregation and Composition

• When implementing a class aimed to demonstrate an aggregation object relationship, do not do this:

Other Key Concepts / Best Practises

Enums (Enumerated Types)

Java Collections Framework

Class Invariants

JUnit Framework

Obligatory Methods

Exceptions