☕ Java Core Masterclass

A comprehensive Java learning project — from beginner fundamentals to senior interview-level deep dives.

Java 21 LTS · Maven · 39 runnable lessons

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🚀 Quick Start

git clone <repository-url>
cd java-core
mvn clean compile

# Run any lesson directly
mvn exec:java -Dexec.mainClass="com.learning.exception.ExceptionErrorInsight"

Requirement	Version
Java	21 LTS
Maven	3.9+

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📚 Learning Roadmap

Follow the levels in order. Each builds on the previous one.

Level 1 — Foundations (Beginner)

Start here if you're new to Java. Learn how the language works from the ground up.

#	Topic	File	What You'll Learn
1	Primitive Types	PrimitiveTypes.java	8 data types, bytes/bits, memory layout
2	Operators	Operators.java	Arithmetic, logical, bitwise, assignment
3	Control Flow	ControlFlow.java	if-else, switch, loops
4	Arrays	ArraySystemInsight.java	Single/multi-dimensional arrays, memory
5	Strings	StringPoolInsight.java	String pool, interning, immutability
6	Stack vs Heap	StackVsHeapInsight.java	Where data lives in memory

Level 2 — Object-Oriented Programming

Understand how Java organizes code into objects.

#	Topic	File	What You'll Learn
7	Classes & Objects	ClassesAndObjects.java	Classes, constructors, this keyword
8	Object Anatomy	ObjectAnatomyInsight.java	Object header, memory layout
9	Object Header	ObjectHeaderInsight.java	Mark word, class pointer, lock bits
10	Method Execution	MethodExecutionInsight.java	Stack frames, method dispatch
11	Records	RecordsDemo.java	Java 14+ immutable data classes

Level 3 — Collections & Memory

How Java stores and manages data structures in memory.

#	Topic	File	What You'll Learn
12	Collections Overview	CollectionsOverview.java	Hierarchy, choosing the right collection, golden rules
13	ArrayList	ArrayListInsight.java	Growth 1.5x, internal Object[], cache-friendly, efficiency tips
14	LinkedList	LinkedListInsight.java	Doubly-linked nodes, Deque API, why ArrayList almost always wins
15	HashSet	HashSetInsight.java	Backed by HashMap, equals/hashCode contract, set operations
16	TreeSet	TreeSetInsight.java	Red-Black Tree, NavigableSet API, floor/ceiling/subSet
17	HashMap Internals	HashMapInsight.java	Buckets, hashing, tree-ification
18	HashMap Deep Dive	HashMapInternals.java	Resize, load factor, collisions
19	TreeMap	TreeMapInsight.java	Sorted map, NavigableMap, range queries, real-world examples
20	LinkedHashMap	LinkedHashMapInsight.java	Insertion/access order, LRU cache in 5 lines
21	Queue & Deque	QueueDequeInsight.java	ArrayDeque circular buffer, PriorityQueue heap, Top-K pattern
22	Concurrent Collections	ConcurrentCollectionsInsight.java	ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue
23	Variable Memory	VariableMemoryInsight.java	Where primitives vs objects live
24	Object Memory Layout	ObjectMemoryLayout.java	Alignment, padding, field ordering

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Level 4 — Senior Deep Dives 🔥

The heart of this project. Each chapter is a comprehensive, interview-ready lesson with JVM internals, ASCII diagrams, runnable demos, and best practices.

#	Topic	File	Key Concepts
25	Exception & Error	ExceptionErrorInsight.java	Exception tables, fillInStackTrace() cost, try-with-resources internals, custom hierarchies
26	Async & Concurrency	AsyncPatternsInsight.java	Thread pools, CompletableFuture, ForkJoinPool, Virtual Threads (Java 21)
27	Volatile & Locks	VolatileLockInsight.java	JMM, happens-before, lock escalation, CAS atomics, deadlock prevention
28	Vavr Concepts	VavrConceptsInsight.java	Option, Either, Try internals, validation pipelines, pattern matching
29	Java Functions	JavaFunctionsInsight.java	java.util.function, type erasure, PECS rule, reading generic code
30	Functional Style	FunctionalStyleInsight.java	Pure functions, Streams internals, Collectors, method references
31	Serialization	SerializationInsight.java	Byte stream format, Serialization Proxy Pattern, Externalizable
32	JDBC & JPA	JdbcJpaInsight.java	PreparedStatement, batch ops, N+1 problem, connection pooling, entity lifecycle
33	RMI	RmiInsight.java	Stub-skeleton, marshalling, dynamic proxy, modern alternatives
34	Design Patterns	DesignPatternsInsight.java	7 patterns with before/after refactoring + code smell guide
35	JVM Internals	JvmInternalsInsight.java	GC algorithms (G1/ZGC), JVM flags, classloading, memory leaks, profiling
36	Architecture	ArchitectureInsight.java	Hexagonal, package-by-feature, SOLID, DTO pattern
37	Testing	TestingInsight.java	JUnit 5, Mockito, AssertJ, Testcontainers, TDD, anti-patterns

Bonus

#	Topic	File	What You'll Learn
38	Sorting Algorithms	SortingAlgorithms.java	Bubble, Selection, Insertion, Merge, Quick
39	Interview Questions	JavaCoreQuestions.java	Common senior interview topics

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📂 Project Structure

src/main/java/com/learning/
├── fundamentals/          # Level 1: Primitives, operators, arrays, strings, memory
│   ├── primitives/
│   ├── operators/
│   ├── controlflow/
│   ├── arrays/
│   ├── strings/
│   ├── memory/
│   └── objects/
├── oop/                   # Level 2: Classes, objects, method dispatch
│   ├── classes/
│   ├── anatomy/
│   └── methods/
├── modern/                # Level 2: Records, sealed classes
├── collections/           # Level 3: All collection types
│   ├── list/              #   ArrayList, LinkedList
│   ├── set/               #   HashSet, TreeSet
│   ├── map/               #   HashMap, TreeMap, LinkedHashMap
│   ├── queue/             #   ArrayDeque, PriorityQueue
│   └── concurrent/        #   ConcurrentHashMap, CopyOnWrite, BlockingQueue
├── memory/                # Level 3: Object layout, variable storage
├── exception/             # Level 4: Exception internals
├── concurrency/           # Level 4: Async, volatile, locks
├── functional/            # Level 4: Vavr, functions, streams
├── serialization/         # Level 4: Serializable internals
├── database/              # Level 4: JDBC & JPA
├── rmi/                   # Level 4: Remote Method Invocation
├── patterns/              # Level 4: Design patterns & refactoring
├── jvm/                   # Level 4: GC, JVM flags, classloading
├── architecture/          # Level 4: Hexagonal, SOLID, packaging
├── testing/               # Level 4: JUnit, Mockito, TDD
├── algorithms/            # Bonus: Sorting algorithms
└── interview/             # Bonus: Interview preparation

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🏃 How to Run

Each file has a main() method. Run any lesson:

# Run a specific lesson
mvn compile exec:java -Dexec.mainClass="com.learning.concurrency.AsyncPatternsInsight"

# Run all tests
mvn test

# Build the project
mvn clean compile

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📖 Recommended Reading

Book	Why
Effective Java — Joshua Bloch	The Java bible. Patterns, pitfalls, best practices
Java Concurrency in Practice — Brian Goetz	Threading done right
Clean Code — Robert C. Martin	Writing maintainable software
Head First Design Patterns	Visual, beginner-friendly pattern intro

🛠 Tools

Tool	Purpose
VisualVM	CPU & memory profiling
JMH	Microbenchmarking
async-profiler	Low-overhead profiling
Eclipse MAT	Heap dump analysis

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License

This project is for educational purposes.