🎯 Java Fullstack Interview Prep — Master Notes

How to use this doc in Notion: Import this .md file (Settings → Import → Text & Markdown). Every ## becomes a Notion heading (collapsible if you toggle it), every > becomes a callout-style quote block, and code fences render as code blocks. Add a “Confidence” tag (🟢/🟡/🔴) next to each heading as you revise.

📌 How to Use These Notes

• Each question has: Short Answer (say this first, 20-30 sec) → Deep Dive (if they probe) → Example → 💡 What Interviewer Is Really Checking.
• Don’t memorize sentences — memorize the structure (Definition → Why → Example → Trade-off). That structure alone gets you through 80% of Java backend interviews.
• Read the 💡 callouts even if you skip everything else — they tell you why the question is asked, which helps you answer follow-ups you haven’t seen before.

1️⃣ Spring & Spring Boot

Hibernate–JPA Structure

Short Answer: JPA is a specification (interfaces like EntityManager, EntityManagerFactory); Hibernate is the most popular implementation of that spec. Spring Data JPA sits on top of both and auto-generates repository implementations.

Deep Dive — the layers:

Your Code
   ↓
Spring Data JPA (Repository interfaces — no SQL written)
   ↓
JPA (spec: EntityManager, @Entity, JPQL)
   ↓
Hibernate (implementation: SessionFactory, Session, HQL, caching)
   ↓
JDBC (driver-level, raw SQL)
   ↓
Database

Example:

public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email); // Spring Data generates the query
}

💡 What they’re checking: Whether you know JPA ≠ Hibernate (many candidates conflate them), and whether you understand why the abstraction exists — portability across ORM vendors (EclipseLink, OpenJPA).

Enabling Native SQL in JPA

Short Answer: Use @Query(nativeQuery = true) on a repository method, or entityManager.createNativeQuery().

Example: