java-expert

☕ Java Expert — Production-Grade Java

Philosophy: Modern Java is not your grandfather's Java. Records, sealed classes, pattern matching, and virtual threads have transformed the language. Write modern Java, not enterprise Java from 2010.

1. When to Use This Skill

• Writing new Java services or libraries
• Reviewing Java code for modern patterns
• Building Spring Boot applications
• Working with JPA/Hibernate
• Configuring Maven or Gradle builds
• Writing tests with JUnit 5

2. Modern Java (17+)

Records Over Boilerplate Classes

// GOOD: Record — immutable, equals/hashCode/toString auto-generated
public record UserResponse(String id, String name, String email, Instant createdAt) {}

// GOOD: Record with validation
public record CreateUserRequest(String name, String email) {
    public CreateUserRequest {
        if (name == null || name.isBlank()) throw new IllegalArgumentException("name is required");
        if (email == null || !email.contains("@")) throw new IllegalArgumentException("invalid email");
    }
}

// BAD: 80-line POJO with getters, setters, equals, hashCode, toString, builder
public class UserResponse {
    private String id;
    private String name;
    // ... 70 more lines of boilerplate
}

Sealed Classes for Domain Modeling

// GOOD: Sealed hierarchy — compiler knows all cases
public sealed interface PaymentResult permits Success, Failed, Pending {
    record Success(String transactionId, BigDecimal amount) implements PaymentResult {}
    record Failed(String reason, String errorCode) implements PaymentResult {}
    record Pending(String pollUrl, Instant expiresAt) implements PaymentResult {}
}

// Pattern matching with exhaustive switch
String message = switch (result) {
    case Success s -> "Paid $%s (tx: %s)".formatted(s.amount(), s.transactionId());
    case Failed f -> "Failed: %s (%s)".formatted(f.reason(), f.errorCode());
    case Pending p -> "Pending — check %s".formatted(p.pollUrl());
};