レコード、クラス、Lombok の比較

// Traditional class — lots of boilerplate
class ProductOld {
    private final String id;
    private final String name;
    private final double price;

    public ProductOld(String id, String name, double price) {
        this.id = id; this.name = name; this.price = price;
    }
    public String getId() { return id; }
    public String getName() { return name; }
    public double getPrice() { return price; }

    @Override public boolean equals(Object o) { /* ... */ }
    @Override public int hashCode() { /* ... */ }
    @Override public String toString() { /* ... */ }
}

// Record — one line, same functionality
record Product(String id, String name, double price) {}

// Usage is almost identical except accessors use component names (no get prefix)
Product p = new Product("P001", "Laptop", 999.99);
System.out.println(p.id());     // P001 (not getId())
System.out.println(p.name());   // Laptop
System.out.println(p.price());  // 999.99
System.out.println(p);          // Product[id=P001, name=Laptop, price=999.99]

// Lombok @Value — immutable (closest to record)
import lombok.Value;

@Value
public class LombokProduct {
    String id;
    String name;
    double price;
    // Lombok generates: all-args constructor, getters, equals, hashCode, toString
    // Note: accessors follow getX() naming convention (getPrice(), not price())
}

// Lombok @Data — mutable
import lombok.Data;
@Data
public class MutableProduct {
    private String id;
    private String name;
    private double price;
    // Generates: getters, setters, equals, hashCode, toString, no-args constructor
}

// Perfect record use cases:
record Coordinates(double lat, double lon) {}
record JwtClaims(String subject, String role, long expiresAt) {}
record PageRequest(int page, int size, String sortBy) {}
record ErrorResponse(int code, String message, String path) {}

// These are all simple data carriers with no mutable state

// Lombok @Builder — great for complex object construction
import lombok.*;

@Builder
@Value
public class EmailMessage {
    String to;
    String from;
    String subject;
    String body;
    List<String> attachments;
}

EmailMessage msg = EmailMessage.builder()
    .to("user@example.com")
    .from("noreply@app.com")
    .subject("Welcome!")
    .body("Thank you for signing up")
    .attachments(List.of())
    .build();

// Plain class — when behavior dominates over data
class ShoppingCart {
    private final List<CartItem> items = new ArrayList<>();
    private final String customerId;

    public ShoppingCart(String customerId) {
        this.customerId = customerId;
    }

    public void addItem(CartItem item) { items.add(item); }
    public void removeItem(String sku) { items.removeIf(i -> i.sku().equals(sku)); }
    public double total() { return items.stream().mapToDouble(CartItem::lineTotal).sum(); }
    public boolean isEmpty() { return items.isEmpty(); }
}

// Record for the immutable event data
record UserEvent(String userId, String action, java.time.Instant timestamp) {
    public static UserEvent now(String userId, String action) {
        return new UserEvent(userId, action, java.time.Instant.now());
    }
}

// Class for the mutable event store
class EventLog {
    private final List<UserEvent> events = new ArrayList<>();

    public void record(String userId, String action) {
        events.add(UserEvent.now(userId, action));
    }

    public List<UserEvent> eventsFor(String userId) {
        return events.stream()
            .filter(e -> e.userId().equals(userId)).toList();
    }
}

// Spring Boot REST controller using records
import org.springframework.web.bind.annotation.*;

record CreateProductRequest(String name, double price, String category) {}
record ProductResponse(long id, String name, double price, String category) {}

@RestController
@RequestMapping("/api/products")
class ProductController {
    @PostMapping
    public ProductResponse create(@RequestBody CreateProductRequest req) {
        // Jackson automatically deserializes JSON into the record
        long id = productService.save(req);
        return new ProductResponse(id, req.name(), req.price(), req.category());
    }
}

// All three compile to approximately equivalent bytecode:
// - Records: compiler-generated, always immutable
// - Lombok @Value: annotation-processor-generated, always immutable
// - Plain class: hand-written

// The key runtime difference:
// Records: accessor is a method matching field name (price() not getPrice())
// Lombok/Class: accessor follows JavaBeans (getPrice())

// Jackson compatibility:
// Records: work natively in Jackson 2.12+ with no config
// Lombok: work with @JsonProperty or jackson-databind-lombok module