java-patterns

Java Patterns Skill

Modern Java for production applications. Focuses on Java 17+ features and Spring Boot conventions.

When to Activate

Activate when:

• Writing new Java services or libraries
• Reviewing Java code for correctness and modern idiom
• Designing API layers, service classes, or data access
• Troubleshooting Spring Boot configuration or JPA queries
• Setting up testing infrastructure

Modern Java Features (17+)

Records — Immutable Data Carriers

Records eliminate boilerplate for data classes. Use them for DTOs, value objects, and command/query objects.

// Compiler generates constructor, getters, equals, hashCode, toString
public record Point(double x, double y) {}

// Custom constructor for validation
public record EmailAddress(String value) {
    public EmailAddress {
        Objects.requireNonNull(value);
        if (!value.contains("@")) {
            throw new IllegalArgumentException("invalid email: " + value);
        }
        value = value.toLowerCase();
    }
}

// Records can implement interfaces
public record PageRequest(int page, int size) implements Serializable {
    public PageRequest {
        if (page < 0) throw new IllegalArgumentException("page must be >= 0");
        if (size < 1 || size > 100) throw new IllegalArgumentException("size must be 1-100");
    }

    public int offset() { return page * size; }
}

Sealed Classes — Closed Hierarchies

Use sealed classes to model a fixed set of variants. The compiler enforces exhaustive pattern matching.

public sealed interface PaymentResult
    permits PaymentResult.Success, PaymentResult.Declined, PaymentResult.Error {

    record Success(String transactionId, BigDecimal amount) implements PaymentResult {}
    record Declined(String reason) implements PaymentResult {}
    record Error(Throwable cause) implements PaymentResult {}
}

// Pattern matching ensures all cases are handled
String message = switch (result) {
    case PaymentResult.Success s  -> "Charged " + s.amount();
    case PaymentResult.Declined d -> "Declined: " + d.reason();
    case PaymentResult.Error e    -> "Error: " + e.cause().getMessage();
};