Interview-focused Java 21 preparation strategy

✅ PHASE 1: Mastering Java OOP with Java 21 Features

🔹 Core OOP Concepts

• Encapsulation, Inheritance, Polymorphism, Abstraction

• Interfaces vs Abstract Classes

• Method Overriding vs Overloading

• Composition over inheritance

• UML to Code
  

🔹 Java 21 Enhancements to Integrate

Concept	Java 21 Feature	Example/Usage
Class Hierarchies	✅ Sealed Classes	Control which classes can extend a superclass. Great for maintainable hierarchies.
Boilerplate Reduction	✅ Records	Immutable data classes. Use in DTOs or event systems.
Pattern Matching	✅ Pattern Matching for switch	Cleaner switch statements in polymorphic object trees.

🧠 Activities:

• Redesign traditional OOP class models using sealed, record, and switch.

• Convert legacy classes to sealed-record combinations.
  

✅ PHASE 2: Java 21 Functional & Concurrency Concepts

🔹 Functional Programming Features

• Lambdas, Streams API

• Method references

• Collectors & grouping

• Optional

🔹 Java 21 Additions

• Enhanced Pattern Matching

• String Templates (Preview) – useful for logging/debugging
  

🧠 Activities:

• Refactor forEach/loops with streams

• Build a UserService that returns filtered/search results using functional features.

✅ PHASE 3: Modern Concurrency & Reactive Foundations

🔹 Java 21 Structured Concurrency (NEW!)

• StructuredTaskScope for handling task hierarchies.

• Simplifies concurrent task cancellation/management.

• Ideal replacement for managing async tasks manually via ExecutorService.
  

🔹 Virtual Threads (Project Loom)

• Revolutionary change! Lightweight threads ideal for scalable, reactive-like code using imperative style.

Thread.startVirtualThread(() -> {

// Simulate DB/API call

}); 🔸 Use this instead of blocking Thread or thread pools for IO-bound tasks.

✅ PHASE 4: Reactive Programming Using Spring & Project Reactor

🔹 Core Concepts

• Mono vs Flux

• Publisher/Subscriber

• Cold vs Hot streams

• Backpressure

• Schedulers
  

🔹 Hands-On: Spring WebFlux (Reactive Stack)

• Build reactive endpoints: /orders, /stream/items, etc.

• Connect with MongoDB using reactive drivers

• Use Flux.interval for simulating live streams
  

🔹 Simulate Interview Case:

“Design a Reactive Streaming API for Live Sports Scores.”

✅ PHASE 5: Mock Interviews & System Design

🔹 Focus Areas

• Refactoring imperative to reactive

• Explaining design choices (sealed classes vs inheritance)

• Exception handling in reactive pipelines

• Threading model of virtual threads vs traditional

• Performance: when to use virtual threads vs reactive streams
  

🔹 Sample Questions

• "How would you manage 10k concurrent HTTP requests in Java 21?"

• "Can you model a payment processing pipeline using reactive programming?"

• "Why use sealed classes instead of enums in modern Java?"

• "Compare virtual threads and reactive streams for IO-bound tasks."
  

✅ PHASE 6: Capstone Project (with GitHub Portfolio)

Project Idea: “Real-time Stock Trading System”

• Reactive endpoints using Spring WebFlux

• Java 21 features: records, sealed classes, virtual threads for price simulation

• API to subscribe to stock updates (Flux stream)

• Admin panel secured with Spring Security