Optional のベストプラクティス

Optionalのベストプラクティス

行うべきこと: 戻り値の型としてOptionalを使用する

import java.util.Optional;

// Good: signals to caller that result might be absent
public Optional<User> findByUsername(String username) { ... }
public Optional<Config> getConfig(String key) { ... }
public Optional<String> getHeader(String name) { ... }

// These are all cases where absence is a normal outcome,
// not an exceptional condition

避けるべきこと: フィールドにOptionalを使用する

// BAD
class User {
    private Optional<String> middleName; // wrong
}

// GOOD: use null for absent optional field
class User {
    private final String middleName; // null = absent
    public Optional<String> middleName() {
        return Optional.ofNullable(middleName); // expose as Optional in getter
    }
}
User u = new User(null); // null field
System.out.println(u.middleName().isPresent()); // false

避けるべきこと: メソッドの引数にOptionalを使用する

// BAD
void sendNotification(String message, Optional<String> subject) { ... }
// caller must write: sendNotification("Hello", Optional.of("News"))

// GOOD: overloading
void sendNotification(String message) { sendNotification(message, null); }
void sendNotification(String message, String subject) {
    // subject may be null
}
// caller writes: sendNotification("Hello", "News") — clean

避けるべきこと: コレクションにOptionalを使用する

// BAD: List of Optionals
List<Optional<User>> results = ...; // meaningless empty Optionals

// GOOD: List of present values only
List<User> presentUsers = maybeUsers.stream()
    .flatMap(Optional::stream)
    .collect(Collectors.toList());

// BAD: Map with Optional values
Map<String, Optional<String>> config = ...;

// GOOD: just use null or omit the key for absent values
Map<String, String> config2 = ...;
String value = config2.getOrDefault("key", "default");

行うべきこと: 計算コストの高いデフォルト値にはorElseGetを使用する

import java.util.Optional;

// BAD: buildDefault() always called, even when value is present!
String config = getConfig("timeout").orElse(buildDefault());

// GOOD: supplier called lazily only when empty
String config2 = getConfig("timeout").orElseGet(() -> buildDefault());

// Or with method reference:
String config3 = getConfig("timeout").orElseGet(this::buildDefault);

// For cheap literals, orElse is fine
String name = getConfig("name").orElse("default");

行うべきこと: 必須の値にはorElseThrowを使用する

import java.util.Optional;

// Good: throw when absence is an error
User user = userRepository.findById(userId)
    .orElseThrow(() -> new UserNotFoundException("User not found: " + userId));

// Good: in validation code
String apiKey = Optional.ofNullable(System.getenv("API_KEY"))
    .orElseThrow(() -> new IllegalStateException(
        "API_KEY environment variable is required"));

System.out.println("Key found: " + apiKey.length() + " chars");

アンチパターン: isPresent + get()

import java.util.Optional;

Optional<String> opt = findValue();

// BAD: isPresent() + get() — no improvement over null check
if (opt.isPresent()) {
    String val = opt.get();
    process(val);
}

// GOOD: use the functional API
opt.ifPresent(this::process);

// GOOD: transform and return
return opt.map(this::process).orElse(null);

// GOOD: provide a default
String result = opt.orElse("default");

連結のベストプラクティス

import java.util.Optional;

record User(long id, String plan, boolean active) {}

static Optional<User> findUser(long id) { return Optional.empty(); }

String result = findUser(42L)
    .filter(User::active)           // keep only active users
    .filter(u -> "PRO".equals(u.plan())) // keep only PRO users
    .map(u -> "Welcome, Pro user #" + u.id()) // transform
    .orElse("Upgrade to Pro!");     // fallback

System.out.println(result); // Upgrade to Pro!

サービス層でのOptional

import java.util.Optional;

class OrderService {
    public Optional<OrderSummary> getOrderSummary(long orderId) {
        return orderRepo.findById(orderId)
            .filter(o -> !o.isCancelled())
            .map(this::toSummary);
    }

    public OrderSummary requireOrderSummary(long orderId) {
        return getOrderSummary(orderId)
            .orElseThrow(() -> new OrderNotFoundException(orderId));
    }

    private OrderSummary toSummary(Order o) { return new OrderSummary(o); }
}
// API callers choose: Optional (handle absence) or require (exception)

Optionalとnull安全性のツール

// Annotation-based: declares intent for in-class nulls
import org.jetbrains.annotations.*;

class ProductService {
    @Nullable  // may return null — for framework/bean use
    public String getCachedName(String id) { return cache.get(id); }

    // Optional — for service API that handles absence
    public Optional<Product> findProduct(String id) {
        return repo.findById(id);
    }
}
// Use @Nullable for fields/parameters; Optional for return types

クイックチェック

if (opt.isPresent()) {
    process(opt.get());
}

まとめ: Optionalのベストプラクティス