Factory and Abstract Factory

Centralise object creation when the concrete type should be a detail.

Intermediate10 min read

Read these first

When to Use Which Pattern

Why this matters

Simple factory — detailed pointwise breakdown in the section below.
Abstract factory — detailed pointwise breakdown in the section below.
CampusOS notifications — detailed pointwise breakdown in the section below.
Testing with factories — detailed pointwise breakdown in the section below.
Patterns are forces and solutions — name the force before the pattern.

Creational

Factorychoose implementation

Builderstep-by-step assembly

Structural

Decoratorstack behaviour

Facadesimple entry point

Behavioral

Strategyswap algorithm

Observernotify subscribers

Statebehaviour by mode

Pick the family first, then the specific pattern.

Overview

Centralise object creation when the concrete type should be a detail. Study each section below in order — every list is interview-ready reference material.

Simple factory

Points to cover

Definition — what Simple factory means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Abstract factory

Points to cover

Definition — what Abstract factory means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

CampusOS notifications

Points to cover

Definition — what CampusOS notifications means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Testing with factories

Points to cover

Definition — what Testing with factories means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Java

// CampusOS — apply "Simple factory" on paper first, then in code
public final class FactoryAndAbstractFactoryExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Simple factory — review this slice before mock interviews.
Abstract factory — review this slice before mock interviews.
CampusOS notifications — review this slice before mock interviews.
Testing with factories — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.