Must Prepare Topic Before Attending Any Java Backend Interview

1. Java Core

• What are the differences between JDK, JRE, and JVM?
• How does the JVM manage memory?
• Explain Heap vs Stack.
• What causes OutOfMemoryError?
• What causes StackOverflowError?
• How does Garbage Collection work?
• What GC algorithms are available?
• Explain equals() vs ==.
• Why should hashCode() be overridden with equals()?
• Explain immutable classes.
• How does String Pool work?
• Explain volatile.
• What is synchronization?
• What is Reentrant Lock?
• Explain Atomic classes.
• Explain ExecutorService.
• Callable vs Runnable.
• Future vs CompletableFuture.
• How does ConcurrentHashMap work?
• Explain fail-fast vs fail-safe iterators.

---

2. Java 8

• Functional Interface
• Lambda Expressions
• Method References
• Stream API
• Parallel Stream
• Optional
• Collectors
• GroupingBy
• PartitioningBy
• FlatMap
• Reduce
• Default Methods
• Date & Time API

Coding questions:

• Find duplicate elements
• Count frequency
• Top N salaries
• Second highest salary
• Group employees by department
• Merge lists
• Remove duplicates
• Find longest string

---

3. Spring Boot

• Why Spring Boot?
• Spring Boot Auto Configuration
• Starter Dependencies
• Spring Boot Lifecycle
• Bean Lifecycle
• Bean Scope
• @Component vs @Bean
• @Autowired vs Constructor Injection
• @Value vs @ConfigurationProperties
• Profiles
• External Configuration
• Actuator
• Spring Boot Logging
• Exception Handling
• Validation
• Filters
• Interceptors

---

4. Spring Security + JWT

• Authentication vs Authorization
• Security Filter Chain
• JWT Flow
• Stateless Authentication
• Refresh Token
• Access Token
• OAuth2
• CORS
• CSRF
• Password Encoding
• Method Security
• Custom Authentication Filter

---

5. JPA & Hibernate

You’ve already covered most of these, but revise:

• Entity States
• Persistence Context
• First-Level Cache
• Second-Level Cache
• Lazy vs Eager
• N+1 Problem
• Cascade Types
• Orphan Removal
• Fetch Join
• JPQL
• Criteria API
• Hibernate Filters
• Optimistic Locking
• Pessimistic Locking
• Dirty Checking
• Transactions
• Batch Processing

---

6. SQL

• Joins
• Indexes
• Composite Index
• Clustered vs Non-clustered Index
• Execution Plan
• Explain Analyze
• ACID
• Isolation Levels
• Deadlock
• Locking
• MVCC
• Window Functions
• CTE
• Stored Procedure
• Trigger
• Pagination

Coding:

• Nth Highest Salary
• Duplicate Records
• Running Total
• Rank
• Dense Rank
• Department-wise highest salary
• Recursive queries

---

7. Microservices

• Why Microservices?
• Monolith vs Microservices
• Service Discovery
• API Gateway
• Config Server
• Circuit Breaker
• Retry
• Bulkhead
• Rate Limiting
• Distributed Transactions
• Saga Pattern
• Event-driven Architecture
• Synchronous vs Asynchronous Communication
• REST vs Messaging
• Idempotency
• API Versioning

---

8. Docker

• Docker Architecture
• Dockerfile
• Image vs Container
• Volume
• Network
• Multi-stage Build
• Docker Compose
• Health Check

---

9. Kubernetes

• Pod
• ReplicaSet
• Deployment
• Service
• Ingress
• ConfigMap
• Secret
• Namespace
• Persistent Volume
• Liveness Probe
• Readiness Probe
• HPA
• Rolling Update
• Rollback

---

10. Observability (Very Hot Topic)

Since we discussed this today.

• What is Observability?
• Monitoring vs Observability
• Three Pillars
• Metrics
• Logs
• Traces
• Distributed Tracing
• What is OpenTelemetry?
• OpenTelemetry Architecture
• OpenTelemetry Collector
• Java Agent
• Custom Instrumentation
• Prometheus
• Grafana
• Loki
• Elasticsearch
• Jaeger
• Grafana Tempo
• Trace ID
• Span ID
• PromQL
• Alerting

Scenario Questions:

• API is slow. How do you investigate?
• Database deadlock occurred.
• CPU is high.
• Memory leak.
• Service A calling Service B failed.
• Kafka consumer lag increased.

---

11. Kafka

• Kafka Architecture
• Topic
• Partition
• Offset
• Consumer Group
• Rebalancing
• Replication
• Producer Acknowledgement
• ISR
• Idempotent Producer
• Exactly Once
• Retry
• DLQ

---

12. Redis

• Why Redis?
• Cache Aside
• Write Through
• Write Back
• Redis Data Types
• TTL
• Pub/Sub
• Distributed Lock
• Cache Eviction

---

13. System Design

Design:

• URL Shortener
• Parking Lot
• WhatsApp
• BookMyShow
• Food Delivery
• Hotel Booking
• Payment System
• Notification System

Concepts:

• Load Balancer
• CDN
• Caching
• Sharding
• Replication
• Partitioning
• CAP Theorem
• Consistent Hashing
• Database Scaling

---

14. DevOps

• CI/CD
• Jenkins
• GitHub Actions
• Argo CD
• GitOps
• Blue-Green Deployment
• Canary Deployment
• Rolling Update

---

15. AWS Basics

• EC2
• S3
• RDS
• IAM
• VPC
• ELB
• Auto Scaling
• CloudWatch
• ECS
• EKS

---

16. Scenario-Based Questions (Most Important)

These are very common in interviews:

• Your application suddenly becomes slow. How will you investigate?
• Memory usage keeps increasing every day. What will you do?
• CPU utilization is 100%. How will you debug it?
• One microservice is timing out. Where do you start?
• Database deadlocks are occurring frequently.
• Kafka messages are not being consumed.
• One service is returning 500 errors.
• A deployment failed after release. How will you rollback?
• How do you debug production issues without direct server access?
• How do you identify which microservice is causing latency?
• How would you design a highly available payment system?
• What happens when one service is down in a microservice architecture?
• How do you prevent duplicate order creation?
• How would you handle distributed transactions?

---

17. Coding (DSA)

For product companies, don’t skip this.

• Arrays
• Strings
• HashMap
• Stack
• Queue
• Linked List
• Binary Tree
• BST
• Heap
• Graph
• BFS
• DFS
• Sliding Window
• Two Pointer
• Binary Search
• Dynamic Programming (basic)