Optional<String> nonNullOpt = Optional.of("value"); // 值必須非空（否則拋NPE）
Optional<String> nullableOpt = Optional.ofNullable(getNullableValue()); // 接受可能為null的值
Optional<String> emptyOpt = Optional.empty();       // 顯式表示空值

String userId = "user123";
Optional<User> user = userRepository.findById(userId);
// 傳統(tǒng)寫法：多層 null 檢查
if (user != null) {
    Address address = user.getAddress();
    if (address != null) {
        return address.getCity();
    }
}
return "Unknown";
// Optional + map 鏈式寫法
return user.map(User::getAddress)
           .map(Address::getCity)
           .orElse("Unknown");

// 假設(shè) getAddress() 返回 Optional<Address>
return user.flatMap(User::getAddress)
           .map(Address::getCity)
           .orElse("Unknown");

String value = optional.orElse("default");

String value = optional.orElseGet(() -> computeExpensiveDefault());

Optional<User> adultUser = user.filter(u -> u.getAge() >= 18);

User user = userOptional.orElseThrow(() -> new UserNotFoundException(userId));

optional.ifPresent(value -> System.out.println("Found: " + value));

optional.ifPresentOrElse(
    value -> System.out.println("Found: " + value),
    () -> System.out.println("Not found")
);

List<String> cities = users.stream()
    .map(User::getAddress)
    .flatMap(opt -> opt.stream()) // 過濾掉空的 Optional
    .map(Address::getCity)
    .collect(Collectors.toList());

// ? 不要這樣做！
public void process(Optional<String> data) { ... }
private Optional<String> name;

if (optional.isPresent()) {
    String value = optional.get(); // ? 直接暴露空值風(fēng)險
}

Optional<Optional<String>> nested = Optional.of(Optional.of("value"));

Optional<String> result = firstOptional.or(() -> secondOptional);