java-expert

name	java-expert
description	Core Java patterns, modern Java features (JDK 17+), best practices
keywords	["java","jdk","stream","lambda","record","sealed","pattern matching","virtual threads","collections","generics","concurrency"]
filePatterns	["*.java"]
frameworks	["java","jdk"]
tokenCount	2000
version	1.0.0

Java Expert Skill

Modern Java development patterns for JDK 17+ enterprise applications. Think, don't copy. Adapt patterns to context.

🎯 Selective Reading Rule

Read ONLY files relevant to the request! Check the content map, find what you need.

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📑 Content Map

File	Description	When to Read
modern-java.md	Records, sealed classes, pattern matching	New Java 17+ features
streams.md	Stream API patterns, collectors	Data processing
concurrency.md	Virtual threads, CompletableFuture	Async operations
collections.md	Collection patterns, immutability	Data structures
exceptions.md	Error handling, custom exceptions	Exception design
testing.md	JUnit 5, Mockito patterns	Unit testing

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🔑 Core Principles

1. Modern Java Features (JDK 17+)

// Records for immutable data
public record User(String id, String name, String email) {
    // Compact constructor for validation
    public User {
        Objects.requireNonNull(id, "id cannot be null");
        Objects.requireNonNull(email, "email cannot be null");
    }
}

// Sealed classes for domain modeling
public sealed interface PaymentMethod 
    permits CreditCard, BankTransfer, Wallet {
}

public record CreditCard(String number, String cvv) implements PaymentMethod {}
public record BankTransfer(String iban) implements PaymentMethod {}
public record Wallet(String walletId) implements PaymentMethod {}

// Pattern matching
public String processPayment(PaymentMethod method) {
    return switch (method) {
        case CreditCard cc -> processCreditCard(cc);
        case BankTransfer bt -> processBankTransfer(bt);
        case Wallet w -> processWallet(w);
    };
}

2. Stream API Best Practices

// ✅ GOOD: Clear, readable pipeline
List<UserDto> activeUsers = users.stream()
    .filter(User::isActive)
    .filter(u -> u.getCreatedAt().isAfter(cutoffDate))
    .map(UserDto::fromEntity)
    .sorted(Comparator.comparing(UserDto::name))
    .toList();  // JDK 16+

// ✅ GOOD: Grouping with downstream collector
Map<Department, List<Employee>> byDept = employees.stream()
    .collect(Collectors.groupingBy(
        Employee::getDepartment,
        Collectors.filtering(
            Employee::isActive,
            Collectors.toList()
        )
    ));

// ❌ BAD: Side effects in stream
users.stream()
    .forEach(u -> u.setProcessed(true));  // Side effect!

// ✅ GOOD: Map and collect for transformations
List<User> processed = users.stream()
    .map(u -> u.withProcessed(true))  // Immutable
    .toList();

3. Virtual Threads (JDK 21+)

// ✅ GOOD: Virtual threads for I/O-bound tasks
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<Result>> futures = tasks.stream()
        .map(task -> executor.submit(() -> processTask(task)))
        .toList();
    
    List<Result> results = futures.stream()
        .map(this::getFutureSafely)
        .toList();
}

// ✅ GOOD: Structured concurrency (JDK 21+)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Supplier<User> user = scope.fork(() -> fetchUser(userId));
    Supplier<List<Order>> orders = scope.fork(() -> fetchOrders(userId));
    
    scope.join().throwIfFailed();
    
    return new UserProfile(user.get(), orders.get());
}

4. Null Safety

// ✅ GOOD: Use Optional for potentially absent values
public Optional<User> findById(String id) {
    return Optional.ofNullable(repository.findById(id));
}

// ✅ GOOD: Optional chain
String city = user.flatMap(User::getAddress)
    .map(Address::getCity)
    .orElse("Unknown");

// ❌ BAD: Optional.get() without check
String name = findById(id).get();  // Throws if empty!

// ✅ GOOD: Defensive alternatives
String name = findById(id)
    .map(User::getName)
    .orElseThrow(() -> new UserNotFoundException(id));

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✅ Decision Checklist

Before writing Java code:

• Using JDK 17+ features where appropriate?
• Records for data classes?
• Sealed interfaces for closed hierarchies?
• Optional for nullable returns?
• Stream API for collections?
• Virtual threads for I/O operations?
• Immutability by default?

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❌ Anti-Patterns

Anti-Pattern	Why Bad	Better Approach
Getters/Setters everywhere	Breaks encapsulation	Records or builder pattern
Null returns	NPE risk	Optional or empty collection
Checked exceptions for control flow	Performance, readability	Return types or runtime exceptions
Raw types	Type safety	Proper generics
Thread.sleep() for timing	Unreliable	ScheduledExecutor or virtual threads

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🔗 Related Skills

Need	Skill
Build tools	@[skills/maven-gradle]
Unit testing	@[skills/testing-junit]