How to prepare for a Java developer interview?

Java interviews at product companies cover 4 areas. Here's exactly what to prepare:

**1. Core Java (always asked):**
- OOPs: inheritance, polymorphism, abstraction, encapsulation
- Collections: ArrayList vs LinkedList, HashMap internals, TreeMap, HashSet
- Multithreading: synchronized, volatile, ExecutorService, deadlock
- Java 8+: streams, lambda, Optional, functional interfaces
- Exception handling, generics, design patterns (Singleton, Factory, Observer)

**2. Spring Boot:**
- Dependency injection, IoC container
- REST API creation (@RestController, @GetMapping, @PostMapping)
- Spring Data JPA, @Transactional
- Spring Security basics

**3. Database:**
- SQL joins, GROUP BY, indexes
- Hibernate/JPA — N+1 problem, lazy vs eager loading

**4. DSA (Data Structures & Algorithms):**
- Arrays, strings, linked lists
- Binary search, sorting
- Trees, graphs (BFS/DFS)
- Dynamic programming (basics)

**Interview format:** 2-3 rounds technical + 1 system design for senior roles.