DTO vs Entity Separation

Keep transport shapes separate from domain models.

Intermediate9 min read

Read these first

Layered Architecture in Practice

Why this matters

Entity — detailed pointwise breakdown in the section below.
DTO — detailed pointwise breakdown in the section below.
Mapping — detailed pointwise breakdown in the section below.
Leak prevention — detailed pointwise breakdown in the section below.
Production Java structure interviewers expect in backend LLD.

Dependency direction

Controller / FacadeHTTP or API entry

Serviceuse cases, orchestration

Domainentities, rules, invariants

Repositorypersistence port

Dependencies point inward — domain never imports infrastructure.

Overview

Keep transport shapes separate from domain models. Study each section below in order — every list is interview-ready reference material.

Entity

Points to cover

Definition — what Entity 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

DTO

Points to cover

Definition — what DTO 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

Mapping

Points to cover

Definition — what Mapping 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

Leak prevention

Points to cover

Definition — what Leak prevention 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 "Entity" on paper first, then in code
public final class DtoEntitySeparationExample {
    // 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.

Entity — review this slice before mock interviews.
DTO — review this slice before mock interviews.
Mapping — review this slice before mock interviews.
Leak prevention — review this slice before mock interviews.

Test yourself

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