扩展两个Java类而无需组合

I have two configuration classes: MainConfiguration and Configuration each with several fields.

class MainConfiguration {
    public String a;
    public String b;
    public String c;
    // ...
}

class Configuration {
    public String d;
    public String e;
    public String f;
    // ...
}

The goal is to be able to use MainConfiguration, and MainConfiguration + Configuration as two independent classes. Assume that I also cannot:

• 使用组合/依赖注入
• 更改配置类

例如，我可以

class NewConfiguration extends Configuration {
    // copy fields from MainConfiguration
    public String a;
    public String b;
    public String c;
    // ...
}

but that would replicate fields from MainConfiguration. What would be the cleanest way to achieve this goal?