OOP Cheat Sheet

The Four Pillars

Pillar	Purpose	Implementation
Encapsulation	Data hiding + bundling	Private fields, public getters/setters
Abstraction	Hide complexity, show essentials	Abstract classes, interfaces
Inheritance	Code reuse via parent-child	`extends` keyword
Polymorphism	Many forms, same interface	Method overriding, interfaces

Class & Object

class ClassName {
  constructor(params) { /* initialize */ }
  method() { /* behavior */ }
  get property() { /* getter */ }
  set property(val) { /* setter */ }
  static staticMethod() { /* class-level */ }
}
const obj = new ClassName(args);

Access Modifiers

Modifier	Same Class	Subclass	External
`public`	✅	✅	✅
`protected`	✅	✅	❌
`private`	✅	❌	❌

SOLID Principles

Letter	Name	One-Liner
S	Single Responsibility	One class = one job
O	Open/Closed	Extend without modifying
L	Liskov Substitution	Child must work as parent
I	Interface Segregation	Small, focused interfaces
D	Dependency Inversion	Depend on abstractions

Relationships

Type	UML	Lifetime	Example
Association	Simple arrow	Independent	Teacher → Student
Aggregation	Empty diamond ◊	Independent	Department → Teachers
Composition	Filled diamond ◆	Dependent	House → Rooms
Dependency	Dashed arrow	Temporary	OrderService → PaymentService

Abstract Class vs Interface

Aspect	Abstract Class	Interface
Can have implementation?	Yes	No (traditionally)
Can have state?	Yes	No
Multiple inheritance?	No (extends one)	Yes (implements many)

Design Principles

Principle	Meaning
DRY	Don’t Repeat Yourself
KISS	Keep It Simple, Stupid
YAGNI	You Ain’t Gonna Need It
Composition	Favor HAS-A over IS-A
Program to Interface	Depend on abstractions

Key Code Snippets

Inheritance & super

class Child extends Parent {
  constructor(params) { super(params); }
  method() { return super.method() + " extended"; }
}

Private Fields

class BankAccount { #balance = 0; getBalance() { return this.#balance; } }

Static Members

class MathUtils { static PI = 3.14159; static square(x) { return x * x; } }
MathUtils.PI; // 3.14159

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