java中泛型类型的组合(composition of generic types in java)

 假设我正在定义一个类User，它具有扩展Principal的泛型类型 
 
public abstract class User<P extends Principal> {
}
 
 我有一个动作基类，应该与用户参数化： 
 
public abstract class Action<U extends User<Principal>> {
}
 
 这两个类都在muy代码库上。 现在，我想在特定的应用程序上使用它们，所以我为Principal和User创建了一个实现： 
 
public class MyPrincipal implements Principal {
    public String getName() {
        return "Administrator";
    }
}

public class MyUser extends User<MyPrincipal>{
}
 
 一旦我拥有了我的用户，我就想在一个动作上使用它 
 
public class MyAction extends Action<MyUser>{
}
 
 最后一个类不编译。 它抱怨说： 
 
Bound mismatch: The type MyUser is not a valid substitute for the bounded parameter <U extends User<Principal>> of the type Action<U>
 
 我想要的是强制所有扩展MyAction的类都应该使用扩展User的对象进行参数化，并且还要通过实现Principal的对象进行参数化。 我错过了什么？ 

Let's say I'm defining a class User, which has a generic type which extends Principal
 
public abstract class User<P extends Principal> {
}
 
And I have an action base class which should be parameterized with an user:
 
public abstract class Action<U extends User<Principal>> {
}
 
Those two classes are on muy codebase. Now, I want to use them on an specific application, so I create an implementation for Principal and User:
 
public class MyPrincipal implements Principal {
    public String getName() {
        return "Administrator";
    }
}

public class MyUser extends User<MyPrincipal>{
}
 
Once I have my user, I want to use it on an action
 
public class MyAction extends Action<MyUser>{
}
 
This last class does not compile. It complais that :
 
Bound mismatch: The type MyUser is not a valid substitute for the bounded parameter <U extends User<Principal>> of the type Action<U>
 
What I want is to enforce that all classes extending MyAction should be parameterized with an object that extends User, and also is parameterized by an object which implements Principal. What I'm missing?

原文：