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Deep, interview-focused tech notes: concepts, code, edge cases and quick revision.

Java

01OOP Principles 02SOLID Principles 03Object Class 04equals() and hashCode()05== vs equals()06String, StringBuilder & StringBuffer 07Immutable Objects 08Wrapper Classes & Autoboxing 09Enums 10Records (Java 16+)11Sealed Classes (Java 17+)12var Keyword (Java 10+)13Pass by Value in Java

Collection Framework23

01Introduction to Java Collections Framework 02Collection Interface (Deep Dive)03List Interface 04ArrayList 05LinkedList 06Vector & Stack 07Set Interface 08HashSet 09LinkedHashSet 10TreeSet 11Queue Interface 12Deque Interface 13PriorityQueue 14Map Interface 15HashMap 16LinkedHashMap 17TreeMap 18WeakHashMap 19IdentityHashMap 20EnumMap 21ConcurrentHashMap 22HashMap Internals 23Load Factor, Fail-Fast and Fail-Safe

Java 8 & Streams22

01Stream Basics 02Filtering 03Mapping 04Sorting 05distinct(), limit() and skip()06Reduction Operations 07Collectors 08groupingBy() and partitioningBy()09Advanced Collectors 10flatMap()11Real Stream Interview Problems 12Optional 13Primitive Streams 14Generating Streams 15Functional Interfaces 16Method References 17Parallel Streams and Performance 18Custom Collectors 19Spliterator and Stream Internals 20Java 8 Interview Masterclass 21Date and Time API (java.time)22Stream API Cheat Sheet

Exception Handling12

01Exception Handling Fundamentals 02Checked vs Unchecked Exceptions 03try-with-resources 04Custom Exceptions 05Exception Hierarchy 06finally Block 07Suppressed Exceptions 08throw vs throws 09Exception Propagation & Stack Unwinding 10Exception Overriding Rules 11Best Practices & Anti-Patterns 12Exception Handling in Spring Boot

01Introduction to Reflection 02The Class Class 03Three Ways to Obtain a Class Object 04Class Loading and Class.forName() Internals 05The Class API 06Constructors in Reflection 07Fields in Reflection 08Methods in Reflection 09Annotations and Reflection 10Dynamic Proxies

Multithreading19

01Introduction to Multithreading 02Creating Threads 03Thread Lifecycle & Thread Methods 04Race Conditions & synchronized 05Java Memory Model & volatile 06Inter-Thread Communication — wait & notify 07Deadlock, Livelock & Starvation 08Explicit Locks — ReentrantLock, ReadWriteLock & StampedLock 09Atomic Variables & Compare-and-Swap 10The Executor Framework 11ThreadPoolExecutor Internals & Pool Sizing 12CompletableFuture 13Synchronizers — CountDownLatch, CyclicBarrier, Semaphore & Phaser 14Concurrent Collections 15Immutability, Thread Confinement & ThreadLocal 16Fork/Join Framework & Parallel Streams 17Virtual Threads (Java 21)18Concurrency in Spring — @Async, synchronized & @Scheduled 19Concurrency Coding Scenarios

Spring

01How Spring Understands Annotations 02@Component 03@Service 04@Repository 05@Controller 06@RestController 07@RequestMapping 08@GetMapping, @PostMapping, @PutMapping, @DeleteMapping and @PatchMapping 09@PathVariable 10@RequestParam 11@RequestBody 12@ResponseBody 13ResponseEntity 14@ControllerAdvice and @ExceptionHandler 15@RequestHeader, @CookieValue and @RequestAttribute 16@ModelAttribute 17@RequestPart, MultipartFile and File Upload 18Bean Validation 19Dependency Injection 20Bean Lifecycle 21Spring AOP 22Spring Security 23Auto-Configuration and @SpringBootApplication 24Spring Boot Actuator and Production Readiness 25Spring Boot Microservices 26Performance and JVM Tuning 27@Value 28@ConfigurationProperties

Spring Data JPA & Hibernate17

01Introduction to Spring Data JPA 02Repository Methods (Derived Queries)03Derived Query Keywords 04Pagination & Sorting 05@Query & JPQL 06Native SQL Queries 07Entity Mapping Essentials 08Persistence Context & Entity Lifecycle 09Relationships & Cascading 10Fetching Strategies & the N+1 Problem 11Transactions & Locking 12Projections, Specifications & Auditing 13Inheritance, Composite Keys & Collections 14Caching, Connection Pooling & Migrations 15Testing the Data Layer 16Advanced Hibernate Features 17JPA & Hibernate Interview Cheat Sheet

Spring Security & JWT6

01Password Encoding with PasswordEncoder 02BCrypt Password Hashing in Java 03JWT Structure & Signing Algorithms 04JWT Authentication in Spring Boot 05Implementing JWT with JJWT 06JWT Security & Interview Scenarios

Logging (SLF4J & Log4j2)3

01Introduction to Logging 02Log4j2 Architecture & Configuration 03Logging in Spring Boot & Production

Design Patterns

Essential Design Patterns12

01Introduction to Design Patterns 02Factory Method Pattern 03Singleton Pattern 04Builder Pattern 05Strategy Pattern 06Observer Pattern 07Proxy Pattern 08Adapter Pattern 09Decorator Pattern 10Circuit Breaker Pattern 11Saga Pattern 12Design Patterns Cheat Sheet

Microservices

Apache Kafka Masterclass15

01Introduction to Apache Kafka 02Kafka Core Concepts 03Kafka Architecture, KRaft & Replication 04Running Kafka Locally & the CLI 05Kafka Producers 06Kafka Consumers & Consumer Groups 07Offset Management & Rebalancing 08Delivery Semantics & Exactly-Once 09Retention, Log Compaction & Storage 10Kafka with Spring Boot (Spring Kafka)11Error Handling, Retries & Dead-Letter Topics 12Schemas, Serialization & Schema Registry 13Kafka Streams & Kafka Connect 14Kafka in Production — Performance, Monitoring & Security 15Kafka Interview Masterclass

Microservices Fundamentals12

01Introduction to Microservices 02Decomposing into Services & Domain-Driven Design 03Inter-Service Communication 04Service Discovery & Load Balancing 05API Gateway with Spring Cloud Gateway 06Centralized Configuration with Spring Cloud Config 07Resilience Patterns with Resilience4j 08Data Management in Microservices 09Security in Microservices 10Observability — Logs, Metrics & Distributed Tracing 11Deploying Microservices — Docker, Kubernetes & CI/CD 12Microservices Interview Questions & Scenarios

Database

01Database Fundamentals and MySQL Architecture 02DDL and CRUD Operations 03Filtering Data 04Sorting and Built-in Functions 05Conditional Logic and NULL Handling 06Transactions, ACID and TCL 07Week 1 Revision and SQL Problem Solving 08INNER JOIN 09LEFT JOIN 10SELF JOIN 11Many-to-Many Relationships and Junction Tables 12JOIN Optimization and Production-Level JOINs 13GROUP BY and Aggregate Functions 14HAVING and Advanced Group Filtering 15ORDER BY, LIMIT and Ranking Queries 16CASE WHEN, IF(), COALESCE(), IFNULL() and NULL Handling 17MySQL Date and Time Functions 18MySQL String Functions and Text Analytics 19Subqueries 20Correlated Subqueries 21Common Table Expressions (CTEs)22Window Functions 23Advanced Window Functions 24Advanced Analytical Queries 25Keys, Foreign Keys & Constraints 26Database Normalization 27Indexes & Query Performance 28Views & Materialized Views 29Stored Procedures & Functions 30Performance Tuning & Interview Masterclass 31Set Operators — UNION & UNION ALL

System Design

System Design Basics14

01How to Approach System Design 02Back-of-the-Envelope Estimation 03Scalability, Availability & Reliability 04Load Balancing 05Caching & CDNs 06Choosing a Database — SQL vs NoSQL 07Database Replication, Partitioning & Sharding 08CAP Theorem & Consistency Models 09Message Queues & Asynchronous Processing 10Rate Limiting 11API Design & Communication Protocols 12Design: URL Shortener 13Design: Notification System 14System Design Cheat Sheet & Practice

Real-World Scenarios

Java Backend Production Scenarios8

01Production Issues & API Performance 02Database & Transaction Scenarios 03Microservices & Distributed Systems Scenarios 04Kafka & Event-Driven Scenarios 05Idempotency, Concurrency & Duplicates 06Java, JVM & Multithreading Scenarios 07Spring Boot Application Scenarios 08Security & Authentication Scenarios

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Updated 2026-10-16