Java Core

A comprehensive Java learning repository covering core topics for Java 25. Organized by topics with self-contained, runnable examples designed for interview preparation.

Topics

Topic	Description	Progress
concurrency	Multithreading & Concurrency	18/18
collections	Collections Framework	6/6
streams	Stream API	4/4
fp	Functional Programming	6/6
oop	OOP & Modern Classes	6/6
pattern	Pattern Matching	6/6
strings	String API	5/5
generics	Generics	5/5
exceptions	Exception Handling	4/4
io	I/O & Networking	4/4
Total		64/64

Structure

src/main/java/org/nkcoder/
├── concurrency/        # Multithreading & Concurrency
├── collections/        # Collections Framework
├── streams/            # Stream API
├── fp/                 # 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 modern virtual threads.

Modern (Java 21+)

Package	Examples	Concepts
virtual/	VirtualThreadExample	Virtual threads, lightweight concurrency
scoped/	ScopedValueExample	ScopedValue (replaces ThreadLocal)
preview/	StructuredConcurrencyExample	StructuredTaskScope (preview)

Recommended

Package	Examples	Concepts
executors/	ExecutorServiceExample, CompletableFutureExample	ExecutorService, async programming
atomic/	AtomicIntegerExample, AtomicReferenceExample	Atomic classes, CAS operations
concurrent_collections/	ConcurrentHashMapExample, CopyOnWriteListExample	Thread-safe collections

Foundational

Package	Examples	Concepts
thread/	ThreadExample, DaemonThreadExample	Thread creation, lifecycle
synchronization/	SynchronizedExample, VolatileExample, WaitNotifyExample	synchronized, volatile
locks/	ReentrantLockExample, ReadWriteLockExample	ReentrantLock, ReadWriteLock
utilities/	CountDownLatchExample, SemaphoreExample	Synchronization utilities

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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

Stream API for data processing pipelines.

Example	Concepts
StreamBasicsExample	Creating streams, intermediate/terminal operations
StreamCollectorsExample	Collectors, groupingBy, partitioningBy, toMap
StreamAdvancedExample	flatMap, reduce, takeWhile, dropWhile
ParallelStreamExample	Parallel streams, when to use, pitfalls

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Functional Programming

Lambdas, functional interfaces, and FP patterns.

Example	Concepts
LambdaExample	Lambda syntax, method references, effectively final
FunctionalInterfaceExample	Function, Predicate, Consumer, Supplier
OptionalExample	Optional creation, chaining, orElse vs orElseGet
CurryingExample	Currying, partial application, function factories
MemoizationExample	Caching pure functions, lazy computation
MonadPatternsExample	Try monad, Validation, railway-oriented programming

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OOP & Modern Classes

Records, sealed classes, and modern OOP patterns.

Example	Concepts
RecordExample	Record syntax, compact constructors
RecordAdvancedExample	Validation, static methods, interfaces
SealedClassExample	sealed, permits, non-sealed
SealedWithRecordsExample	Algebraic data types pattern
InterfaceEvolutionExample	Default, static, private methods
InheritanceExample	Composition over inheritance

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Pattern Matching

Modern pattern matching (all finalized in Java 21+).

Example	Concepts
InstanceofPatternExample	Pattern matching for instanceof
SwitchExpressionExample	Switch expressions, arrow syntax, yield
SwitchPatternExample	Type patterns in switch, null handling
RecordPatternExample	Deconstructing records, nested patterns
GuardedPatternExample	when clauses, guard conditions
ExhaustiveSwitchExample	Sealed types + switch = exhaustiveness

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Strings

String API with modern methods (Java 11+).

Example	Concepts
StringBasicsExample	Immutability, string pool, intern()
StringMethodsExample	isBlank, strip, lines, repeat, indent
TextBlockExample	Multi-line strings, indentation, escapes
StringBuilderExample	Mutable strings, StringJoiner
StringFormattingExample	format(), formatted(), printf, locales

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Generics

Type-safe programming with generics.

Example	Concepts
GenericsBasicsExample	Generic classes, interfaces, diamond operator
WildcardsExample	?, extends, super - PECS principle
TypeErasureExample	How erasure works, limitations
BoundedTypesExample	Upper/multiple bounds, Comparable
GenericMethodsExample	Static generic methods, type inference

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Exceptions

Exception handling patterns.

Example	Concepts
TryWithResourcesExample	AutoCloseable, suppressed exceptions
CheckedVsUncheckedExample	When to use which, best practices
ExceptionChainingExample	Cause, suppressed, stack traces
CustomExceptionExample	Hierarchies, rich context

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I/O & Networking

Modern file and network I/O (Java 11+).

Example	Concepts
PathAndFilesExample	Path API, Files read/write, attributes
DirectoryOperationsExample	walk, list, find, DirectoryStream
HttpClientExample	HTTP Client API, sync/async requests
HttpClientAdvancedExample	HTTP/2, authentication, cookies

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Running Examples

Each class has a main() method:

./gradlew classes
java -cp build/classes/java/main org.nkcoder.concurrency.virtual.VirtualThreadExample
java -cp build/classes/java/main org.nkcoder.collections.ListExample
java -cp build/classes/java/main org.nkcoder.pattern.SwitchPatternExample

For preview features:

java --enable-preview -cp build/classes/java/main org.nkcoder.concurrency.preview.StructuredConcurrencyExample

Build

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

Requirements

• Java 25+
• Gradle 8+