详解Java Optional正确使用方式和优势(避免空指针异常)

if (user != null && user.getAddress() != null && user.getAddress().getCity() != null) {
    System.out.println(user.getAddress().getCity());
}

Optional<String> name = Optional.of("Tom");           // 不允许为 null
Optional<String> empty = Optional.empty();            // 显式空值
Optional<String> maybeName = Optional.ofNullable(null); // 允许为 null

String name = maybeName.orElse("Default Name");
String name = maybeName.orElseGet(() -> "Generated Name");
String name = maybeName.orElseThrow(() -> new RuntimeException("Name is missing!"));

if (maybeName.isPresent()) {
    System.out.println(maybeName.get());
}

maybeName.ifPresent(System.out::println);

Optional<User> user = getUser();
String city = user
    .map(User::getAddress)
    .map(Address::getCity)
    .orElse("Unknown City");

// 比起返回 null，这种写法更具表达力
public Optional<User> findUserById(Long id) { ... }

String email = user.map(User::getEmail).orElse("no-reply@example.com");

Optional.of(user)
    .map(User::getProfile)
    .map(Profile::getAvatar)
    .ifPresent(System.out::println);

if (user != null && user.getProfile() != null && user.getProfile().getAvatar() != null) {
    return user.getProfile().getAvatar();
} else {
    return "default.png";
}

return Optional.ofNullable(user)
        .map(User::getProfile)
        .map(Profile::getAvatar)
        .orElse("default.png");