Observer Pattern

Decouple event sources from listeners.

Intermediate10 min read

Read these first

Strategy Pattern

Why this matters

Subject/observer — detailed pointwise breakdown in the section below.
Push vs pull — detailed pointwise breakdown in the section below.
CampusOS seat alerts — detailed pointwise breakdown in the section below.
Memory leaks — detailed pointwise breakdown in the section below.
Patterns are forces and solutions — name the force before the pattern.

SubjectnotifyObservers()

Observer A

Observer B

Observer C

Subject notifies subscribers when state changes — decouples publishers from listeners.

Overview

Decouple event sources from listeners. Study each section below in order — every list is interview-ready reference material.

Subject/observer

Points to cover

Definition — what Subject/observer 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

Push vs pull

Points to cover

Definition — what Push vs pull 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 seat alerts

Points to cover

Definition — what CampusOS seat alerts 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

Memory leaks

Points to cover

Definition — what Memory leaks 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 "Subject/observer" on paper first, then in code
public final class ObserverPatternExample {
    // 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.

Subject/observer — review this slice before mock interviews.
Push vs pull — review this slice before mock interviews.
CampusOS seat alerts — review this slice before mock interviews.
Memory leaks — review this slice before mock interviews.

Test yourself

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