基于变量动态确定JSON字段值的教程：以国家-电话区号为例

{
  "BasicData": {
    "country": "United Kingdom"
  },
  "Phone": {
    "Phone prefix": "+44"
  }
}

class Country {
    String displayName;
    String dialingCode;

    public Country(String displayName, String dialingCode) {
        this.displayName = displayName;
        this.dialingCode = dialingCode;
    }

    public String getDisplayName() {
        return displayName;
    }

    public String getDialingCode() {
        return dialingCode;
    }
}

import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

class CountryService {
    private final Map<String, Country> data = new HashMap<>();

    // 使用静态初始化块填充数据，模拟从数据源加载
    public CountryService() {
        data.put("gb", new Country("United Kingdom", "+44"));
        data.put("us", new Country("United States", "+1"));
        data.put("de", new Country("Germany", "+49"));
        // 更多国家数据...
    }

    /**
     * 根据ISO国家代码查找国家信息。
     * @param isoCode 国家的ISO代码（例如 "gb", "us"）
     * @return 包含国家信息的Optional对象，如果未找到则为空。
     */
    public Optional<Country> find(String isoCode) {
        if (isoCode == null || isoCode.trim().isEmpty()) {
            return Optional.empty();
        }
        return Optional.ofNullable(data.get(isoCode.toLowerCase()));
    }
}

// 假设这是您的响应消息结构
class Message {
    private String basicDataCountry;
    private String phonePrefix;

    // 构造函数、Getter方法等
    public Message(String basicDataCountry, String phonePrefix) {
        this.basicDataCountry = basicDataCountry;
        this.phonePrefix = phonePrefix;
    }

    public String getBasicDataCountry() {
        return basicDataCountry;
    }

    public String getPhonePrefix() {
        return phonePrefix;
    }

    // 静态内部类作为构建器
    public static class ResponseBuilder {
        private String basicDataCountry;
        private String phonePrefix;

        public ResponseBuilder basicData(String countryDisplayName) {
            this.basicDataCountry = countryDisplayName;
            return this;
        }

        public ResponseBuilder phone(String dialingCode) {
            this.phonePrefix = dialingCode;
            return this;
        }

        public Message build() {
            return new Message(basicDataCountry, phonePrefix);
        }
    }
}

// 示例控制器或业务逻辑
class Controller {
    private final CountryService countryService; // 假设通过依赖注入初始化

    public Controller() {
        this.countryService = new CountryService(); // 实际项目中应通过DI框架注入
    }

    /**
     * 处理请求，根据ISO国家代码构建响应消息。
     * @param countryIsoCode 客户端传入的国家ISO代码
     * @return 包含国家显示名称和电话区号的Message对象
     * @throws IllegalArgumentException 如果国家ISO代码无效
     */
    public Message handleRequest(String countryIsoCode) {
        Country country = countryService.find(countryIsoCode)
            .orElseThrow(() -> new IllegalArgumentException("Invalid or unsupported country ISO code: " + countryIsoCode));

        return new Message.ResponseBuilder()
            .basicData(country.getDisplayName())
            .phone(country.getDialingCode())
            .build();
    }

    public static void main(String[] args) {
        Controller controller = new Controller();
        try {
            Message ukMessage = controller.handleRequest("gb");
            System.out.println("UK Message: Country=" + ukMessage.getBasicDataCountry() + ", Prefix=" + ukMessage.getPhonePrefix());

            Message usMessage = controller.handleRequest("us");
            System.out.println("US Message: Country=" + usMessage.getBasicDataCountry() + ", Prefix=" + usMessage.getPhonePrefix());

            // 尝试一个不存在的国家代码
            Message invalidMessage = controller.handleRequest("xyz");
            System.out.println("Invalid Message: Country=" + invalidMessage.getBasicDataCountry() + ", Prefix=" + invalidMessage.getPhonePrefix());
        } catch (IllegalArgumentException e) {
            System.err.println("Error: " + e.getMessage());
        }
    }
}