从扩展通用中提取泛型类型 [英] Extract Generic types from extended Generic

问题描述

我试图重构一个类和一组M类扩展任何类的子类，即使我们知道它必须是某种类型的子类。该类型是参数化的，我希望它的参数化类型可用于已经具有M值的子类。

有没有什么办法可以定义这个类而不必包含参数列表中冗余的K和V泛型类型。我希望能够让编译器从任何M被子类映射到的地方推断出它们。

  public abstract class NewParametrized< ; K，V，M延伸SomeParametrized< K，V>> {
 
 public void someMethodThatTakesKAndV（K k1，V v1）{} 
} 
  

换句话说，我希望类声明看起来像这样：

  public class NewParametrized< M扩展SomeParametrized< K，V>> {
  

K和V的类型可以从M的定义中推断出来。

解决方案

问题在于 K 和 V 并没有真正的重复，至少没有比变量声明和对同一变量的使用是重复。

想法这是你声明泛型类型 K 和 V ，然后在定义中使用那些声明的泛型类型泛型类型 M 。因此，您不要多次声明泛型类型 K 或 V ：相反，您声明每个类型一次，然后在别的声明中引用它们。

换一种方式来看，这里泛型类型的数量仍然是三个，即 K ， V 和 M 。 M 碰巧用 K 和 V ，但你不必定义 M 。

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我应该补充说，如果您是类型推断的支持者，您可以查看声明并使用相同的变量作为不必要的重复。但是，如果您习惯使用Java编程（或C或C ++或许多其他语言），那么您可能习惯于在使用它们之前声明变量。

I'm trying to refactor a class and set of subclasses where the M type does extend anything, even though we know it has to be a subclass of a certain type. That type is parametrized and I would like its parametrized types to be available to subclasses that already have values for M.

Is there any way to define this class without having to include the redundant K and V generic types in the parameter list. I'd like to be able to have the compiler infer them from whatever M is mapped to by subclasses.

public abstract class NewParametrized<K, V, M extends SomeParametrized<K, V>> {

    public void someMethodThatTakesKAndV(K k1, V v1) { }
}

In other words, I'd like the class declaration to look something like:

 public class NewParametrized<M extends SomeParametrized<K, V>> {

And K and V's types would be inferred from the definition of M.

解决方案

The problem is that K and V aren't really "repeated", at least not any more than a variable declaration and a use of that same variable are "repetition".

The way to think of this is that you declare the generic types K and V, and then you use those declared generic types in the definition of the generic type M. So, you don't declare the generic type K or V more than once: Rather, you declare each of them once and then refer to them in a declaration of something else.

To look at it another way, the number of generic types here is still three, namely K, V, and M. M happens to be defined in terms of K and V, but you didn't have to define M that way.

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I should add that if you are a proponent of type inference, you might view the declaration and the use of the same variable as unnecessary repetition. However, if you're accustomed to programming in Java (or C, or C++, or many, many other languages), then you're probably accustomed to declaring variables before you use them.

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