✅ Java Mastery Checklist

A comprehensive checklist to master Java backend development from fundamentals to advanced production engineering.

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🟦 1. Java Language Fundamentals

• Understand JVM, JDK, and JRE differences
• Learn primitive types and reference types
• Know Java operators, expressions, and precedence
• Master loops (for, while, do-while)
• Work with arrays, varargs
• Understand String, StringBuilder, and StringBuffer
• Grasp main() method behavior and command-line args
• Learn how memory is managed (stack, heap, metaspace)

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🟦 2. Object-Oriented Programming (OOP)

• Create and use classes, constructors, and objects
• Master inheritance and method overriding
• Understand polymorphism (compile-time, runtime)
• Apply abstraction using abstract classes and interfaces
• Use encapsulation and access modifiers properly
• Differentiate this and super
• Learn overloading (method/constructor)
• Understand static nested, inner, local, and anonymous classes
• Grasp object cloning (shallow vs deep)
• Apply composition over inheritance

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🟦 3. Advanced Language Features

• Use enums effectively
• Understand varargs, static imports
• Apply annotations and custom annotations
• Work with generics, wildcards (<? extends T>, <? super T>)
• Understand wrapper classes and autoboxing
• Differentiate equals(), hashCode(), ==
• Use instanceof, getClass()
• Learn about final, finally, and finalize()
• Use the Reflection API
• Learn Java 14+ Records
• Understand transient, volatile, and static behavior

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🟦 4. Exception Handling

• Understand the exception hierarchy
• Differentiate checked and unchecked exceptions
• Master try-catch-finally, multi-catch
• Create and use custom exceptions
• Use throw vs throws properly
• Learn try-with-resources
• Understand exception chaining and suppression

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🟦 5. Java Collections Framework

• Master core interfaces: List, Set, Map, Queue, Deque
• Work with key implementations (ArrayList, HashMap, etc.)
• Understand LinkedHashMap, TreeSet, WeakHashMap
• Learn differences between Stack, Queue, and Deque
• Use Iterator, ListIterator, and Spliterator
• Differentiate Comparable and Comparator
• Understand fail-fast vs fail-safe iterators
• Use immutable collections effectively
• Use Collections and Arrays utility methods
• Apply custom sorting and comparator chaining

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🟦 6. Concurrency and Multithreading

• Understand thread lifecycle and states
• Use Thread, Runnable, Callable
• Synchronize critical sections using synchronized
• Use wait(), notify(), notifyAll()
• Detect and avoid deadlocks
• Use volatile, ReentrantLock, Semaphore, CountDownLatch
• Work with ExecutorService and thread pools
• Use ThreadLocal for per-thread state
• Use CompletableFuture, Future, and ForkJoinPool
• Master Java 21 Virtual Threads (Project Loom)

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🟦 7. Functional Programming (Java 8+)

• Write lambda expressions
• Work with functional interfaces (Predicate, Function, etc.)
• Use method and constructor references
• Master Stream API (map, filter, reduce, etc.)
• Use Optional for null safety
• Use Collectors and groupingBy
• Implement custom functional interfaces
• Use default and static interface methods

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🟦 8. Java I/O and NIO

• Work with InputStream, OutputStream, Reader, Writer
• Use BufferedReader, BufferedWriter
• Read/write files using File, Path, Files
• Serialize and deserialize objects
• Use Java NIO buffers and channels
• Monitor file changes with WatchService

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🟦 9. JVM Internals and Performance

• Understand class loading process
• Learn runtime data areas (heap, stack, metaspace)
• Understand garbage collection and types (G1, ZGC, etc.)
• Use -Xms, -Xmx, -XX tuning flags
• Profile using jvisualvm, jconsole, Java Flight Recorder
• Understand JIT compiler and escape analysis
• Detect memory leaks and manage ClassLoaders

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🟦 10. Java Security

• Apply secure coding best practices
• Prevent SQL injection, XSS, CSRF
• Work with cryptography (KeyStore, Cipher)
• Implement password hashing (BCrypt, PBKDF2)
• Use secure random numbers
• Understand TLS/SSL in Java apps
• (Legacy) Learn JAAS basics

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🟦 11. Design Patterns & Principles

• Master SOLID principles
• Apply DRY, KISS, YAGNI effectively
• Implement Singleton, Factory, Builder, Strategy
• Use Observer, Decorator, Adapter, Proxy
• Implement Template Method, Composite, Chain of Responsibility
• Understand Dependency Injection
• Use thread-safe and concurrent design patterns

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🟦 12. Testing in Java

• Write unit tests with JUnit 5
• Mock dependencies using Mockito
• Write fluent assertions using AssertJ
• Perform integration tests with Spring Boot
• Use TestContainers for DB/MQ testing
• Analyze code coverage with JaCoCo
• Use parameterized and exception tests
• Understand TDD and BDD basics

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🟦 13. Build Tools and Project Management

• Master Maven: POM, plugins, scopes, BOM
• Understand Gradle: build scripts, tasks
• Manage multi-module projects
• Integrate CI/CD tools (GitHub Actions, Jenkins, GitLab CI)
• Publish artifacts to Nexus, Artifactory

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🟦 14. Modern Java (9–21+)

• Understand Java 9 modules (module-info.java)
• Use var (Java 10) for local inference
• Use HttpClient, String enhancements (Java 11)
• Work with Records (Java 14+)
• Use Sealed Classes (Java 15+)
• Apply pattern matching for instanceof
• Use switch expressions (Java 14/17)
• Learn and apply Virtual Threads (Java 21)

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🟦 15. Backend + Integration Essentials

• Build REST APIs using Spring Boot
• Handle validation and exceptions with @ControllerAdvice
• Secure APIs with Spring Security & JWT
• Document APIs with Swagger / OpenAPI
• Integrate with PostgreSQL, Redis, Kafka, etc.
• Use caching (@Cacheable, Redis, Caffeine)
• Handle async operations (@Async, Executors)
• Use @Scheduled, @EventListener, and messaging queues
• Deploy to Docker, Kubernetes, or cloud
• Optimize startup and memory performance