Java Core

A Java learning repository covering core topics from Java 8 to Java 21. Organized by topics with self-contained, runnable examples.

Topics

Topic	Description	Progress
concurrency	Multithreading & Concurrency	8/10
collections	Collections Framework	6/6
streams	Streams & Functional Programming	7/7
oop	OOP & Modern Classes (Records, Sealed)	0/6
pattern	Pattern Matching	0/6
strings	String API	0/5
generics	Generics	0/5
exceptions	Exception Handling	0/4
io	I/O & Networking	0/3

Structure

src/main/java/org/nkcoder/
├── concurrency/        # Multithreading & Concurrency
├── collections/        # Collections Framework
├── streams/            # Streams & Functional Programming
├── oop/                # OOP & Modern Classes
├── pattern/            # Pattern Matching
├── strings/            # String API
├── generics/           # Generics
├── exceptions/         # Exception Handling
└── io/                 # I/O & Networking

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Concurrency

Thread-safe programming from basics to advanced patterns.

Package	Examples	Concepts
thread/	ThreadExample, DaemonThreadExample	Thread creation, lifecycle, daemon threads
synchronization/	SynchronizedExample, VolatileExample	synchronized, volatile, wait/notify
locks/	ReentrantLockDemo, ReadWriteLockExample	ReentrantLock, ReadWriteLock, Conditions
atomic/	AtomicIntegerExample, AtomicReferenceExample	Atomic classes, CAS operations
executors/	FixThreadPool, ScheduledThreadPool	ExecutorService, thread pools
concurrent_collections/	CopyOnWriteListExample	Thread-safe collections
utilities/	CountDownLatchExample, CyclicBarrierExample	Synchronization utilities
unsafe/	ArrayListUnSafe, HashMapUnSafe	Thread-safety issues demonstration

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Collections

Java Collections Framework with Java 21 features.

Example	Concepts
ListExample	ArrayList, LinkedList, List.of(), List.copyOf()
SetExample	HashSet, TreeSet, LinkedHashSet, Set.of()
MapExample	HashMap, TreeMap, LinkedHashMap, Map.of()
QueueExample	Queue, Deque, PriorityQueue
SequencedCollectionExample	SequencedCollection, reversed(), getFirst/Last()
ImmutableCollectionsExample	Unmodifiable collections, defensive copies

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Streams

Streams API and functional programming.

Example	Concepts
StreamBasicsExample	Creating streams, intermediate/terminal operations
StreamCollectorsExample	Collectors, groupingBy, partitioningBy, toMap
ParallelStreamExample	Parallel streams, when to use, pitfalls
OptionalExample	Optional creation, chaining, orElse vs orElseGet
FunctionalInterfaceExample	Function, Predicate, Consumer, Supplier
LambdaExample	Lambda syntax, method references, effectively final
StreamAdvancedExample	flatMap, reduce, takeWhile, dropWhile

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Running Examples

Each class has a main() method:

./gradlew classes
java -cp build/classes/java/main org.nkcoder.concurrency.thread.ThreadExample
java -cp build/classes/java/main org.nkcoder.collections.ListExample
java -cp build/classes/java/main org.nkcoder.streams.StreamBasicsExample

Build

./gradlew build    # Build the project
./gradlew test     # Run tests
./gradlew clean    # Clean build artifacts

Requirements

• Java 21+
• Gradle 8+