带有泛型的Java F-Bound类型 [英] Java F-Bound types with generics

问题描述

 有没有什么办法可以在java中表达f-边界类型，在调用位置，返回一个通用响应？ >界面Functor< T延伸Functor< T>> 
 public< B> T< B个映射（函数< A，B> fn）; //不会编译，因为类型不匹配
  

我可以使用f-bound类型，如果类型永远不会改变，但在地图的情况下，我需要一个新的类型。有没有一种方法来表达这在Java？

我真正想要的是任何方式，我可以得到像更高类型的东西，即使我知道javac不支持更高的kinded类型。

假设我们有一个 List< A> 并且希望这个接口返回一个列表与LT; B个。但不希望这个接口知道关于 List 的任何信息。

解决方案

读取functor的维基百科定义，这听起来像是你想要定义一个能够从一个类别（Java类型）映射到另一个类型的泛型类型。在上例中，要从 List< A> 映射到 List< B> ，其中类型 A 和 B 是通用的。

如果这是您的目标，然后考虑以下接口来定义 Functor 类型：

 公共界面Functor< CategoryA，CategoryB> {
公共CategoryB地图（CategoryA实例）; 
 
  

这声明 Functor 在两个通用参数类型中输入交易， CategoryA 和 CategoryB ，并且这些参数类型没有限制。它还声明必须实现一个方法 map ，该方法将类型 CategoryA 的对象映射到类型 CategoryB 。

假设根据您的示例，您现在要创建<$ c $的具体实例c> Functor ，它从 List< Integer> 映射到 List< String> 。您可以创建以下类：

  public class IntegerListToStringList实现Functor< List< Integer>，List< String>> {
 @Override 
 public List< String> map（List< Integer> integerList）{
 List< String> stringList = new ArrayList<>（integerList.size（））; 
 for（Integer intValue：integerList）{
 stringList.add（Integer.toString（intValue））; 
} 
 return stringList; 
 
 
 $ / code> 

然后你可以调用这个具体的 Functor 执行如下：



  Functor< List< Integer>，List< String>> functor = new IntegerListToStringList（）; 
 Integer [] intArray = new Integer [] {2，3，5，7，11，13}; 
列表<整数> intList = Arrays.asList（intArray）; 
列表< String> stringList = functor.map（intList）; 
 System.out.println（String list：+ stringList）; 
  

现在，任何期望 Functor< List< Integer>类型参数的方法;，List< String>> 可以接受类型为 IntegerListToStringList 的实例。

Is there any way to express f-bound types in java where at the call site, a generic response is returned?

interface Functor<T extends Functor<T>>
  public <B> T<B> map(Function<A, B> fn); // won't compile because types don't match

I can use f-bound types if the type never changes, but in the case of map, I need a new type. Is there a way to express this in java?

What I am really looking for is any way that I can get something like higher kinds even though I know javac doesn't support higher kinded types.

Lets say we have a List<A> and want this interface to return a List<B>. But don't want this interface to know anything about List.

解决方案

Reading the Wikipedia definition of functor, it sounds like you want to define a generic type which is able to map from one category (Java type) to another. In your example above, to map from a List<A> to List<B> where the types A and B are generic.

If this is your aim, then consider the following interface for the definition of a Functor type:

public interface Functor<CategoryA, CategoryB> {
    public CategoryB map(CategoryA instance);
}

This declares that the Functor type deals in two generic parameter types, CategoryA and CategoryB and no constraints are put on these parameter types. It also declares that a method map must be implemented which maps from an object of type CategoryA to an object of type CategoryB.

Suppose, based on your example, you now want to create a concrete instance of a Functor which maps from List<Integer> to List<String>. You might create the following class:

public class IntegerListToStringList implements Functor<List<Integer>, List<String>> {
    @Override
    public List<String> map(List<Integer> integerList) {
        List<String> stringList = new ArrayList<>(integerList.size());
        for(Integer intValue : integerList) {
            stringList.add(Integer.toString(intValue));
        }
        return stringList;
    }
}

You could then invoke this concrete Functor implementation like so:

Functor<List<Integer>, List<String>> functor = new IntegerListToStringList();
Integer[] intArray = new Integer[] {2, 3, 5, 7, 11, 13};
List<Integer> intList = Arrays.asList(intArray);
List<String> stringList = functor.map(intList);
System.out.println("String list: " + stringList);

Now any method which is expecting a parameter of type Functor<List<Integer>, List<String>> can accept an instance of type IntegerListToStringList.

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