@GetMapping("/users/{id}")
public ResponseEntity<User> getUser(@PathVariable Optional<Long> id) {
    // 鏈?zhǔn)教幚恚捍嬖趇d則查詢，不存在返回400
    return id.map(value -> 
        userService.findById(value)
            .map(ResponseEntity::ok)
            .orElse(ResponseEntity.notFound())
    ).orElse(ResponseEntity.badRequest().build());
}

// 查詢結(jié)果存在時(shí)返回對(duì)象，不存在時(shí)拋出自定義異常
public User getUserOrThrow(Long id) {
    return userRepository.findById(id)
        .orElseThrow(() -> new NotFoundException("User not found"));
}
 
// 安全鏈?zhǔn)秸{(diào)用：查詢、轉(zhuǎn)換、處理空值
public Optional<UserDTO> findUserDTO(Long id) {
    return userRepository.findById(id)
        .map(this::mapToDTO)  // 安全轉(zhuǎn)換DTO
        .filter(dto -> dto.isActive()); // 過濾無效數(shù)據(jù)
}

app.feature.toggle.new-ui=true
@ConfigurationProperties(prefix = "app.feature.toggle")
public class FeatureToggleProperties {
    private Optional<Boolean> newUi = Optional.empty(); // 默認(rèn)空
 
    // Getter/Setter
    public boolean isNewUiEnabled() {
        return newUi.orElse(false); // 配置不存在時(shí)返回默認(rèn)值
    }
}

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(NotFoundException.class)
    public ResponseEntity<Object> handleNotFound(NotFoundException ex) {
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(ex.getMessage());
    }
}

public void logUserActivity(User user) {
    Optional.ofNullable(user)
        .map(User::getName)
        .ifPresent(name -> log.info("User {} logged in", name)); // 僅當(dāng)name存在時(shí)記錄
}

// 反例：返回null，調(diào)用方需自行判斷
public User findUser(Long id) {
    return userRepository.findById(id).orElse(null);
}
 
// 正例：返回Optional，調(diào)用方必須處理空值
public Optional<User> findUser(Long id) {
    return userRepository.findById(id);
}

// 反例：參數(shù)為Optional，增加調(diào)用方負(fù)擔(dān)
void process(Optional<User> user) {
    // ...
}
 
// 正例：使用null或重載
void process(User user) { ... }
void process() { process(null); } // 重載處理無參數(shù)情況

// 反例：直接get()，空值時(shí)崩潰
User user = optionalUser.get();
 
// 正例：安全取值
User user = optionalUser
    .orElse(new User()); // 提供默認(rèn)對(duì)象
User user = optionalUser
    .orElseThrow(() -> new NotFoundException()); // 空值時(shí)拋出異常

List<String> validEmails = users.stream()
    .map(User::getContact)
    .filter(Optional::isPresent)    // 過濾空Optional
    .map(Optional::get)             // 解包Optional
    .map(Contact::getEmail)
    .filter(Objects::nonNull)       // 過濾null郵箱
    .collect(Collectors.toList());

@GetMapping("/search")
public String search(@RequestParam Optional<String> keyword) {
    String query = keyword.orElse("default"); // 無參數(shù)時(shí)使用默認(rèn)值
    return service.search(query);
}

@Configuration
public class OptionalConverterConfig {
    @Bean
    public Converter<String, Optional<String>> optionalConverter() {
        return value -> value == null ? Optional.empty() : Optional.of(value);
    }
}

@Service
public class UserService {
    private final @NonNull UserRepository userRepository; // Lombok保證非空
 
    public Optional<User> findActiveUser() {
        return Optional.ofNullable(userRepository.findActiveUser());
    }
}