从Java的泛型类型推导泛型（编译时错误） [英] Inferring a generic type from a generic type in Java (compile time error)

问题描述

我有一个具有以下签名的静态函数，用于泛型类型 T

  public static< T>列表与LT; T> sortMap（Map< T，Comparable> map）
  

这应该返回带有某些属性的映射键列表。

现在我想传递一个类型为 S

$ b的泛型HashMap $ b

 地图< S，Double>映射
  

在一个泛型类中调用静态函数，该泛型类将映射作为成员变量。 / b>

我在下面列出了一个最小代码示例。

但是，我收到一条错误消息（ S 和 T 都是T，但在我的代码的不同范围内，即 T＃1 = T ， T＃2 = S ）：

 必需：Map< T＃1，Comparable>找到
：Map< T＃2，Double> 
 reason：无法推断类型变量T＃1 
（参数不匹配; Map< T＃2，Double>不能转换为Map< T＃1，Comparable>）
  

如何解决此问题？我很惊讶Java不允许从泛型类型推断泛型。在Java中可以使用什么结构来处理这种更抽象的代码推理？

代码：

  public class ExampleClass< T> {
地图< T，Double>地图; 
 public ExampleClass（）{
 this.map = new HashMap（）; 
} 
 //以下行会产生上述错误
 List< T> sortedMapKeys = UtilityMethods.sortMap（map）; 
} 
 
 public class UtilityMethods {
 public static< T>列表与LT; T> sortMap（Map< T，Comparable> map）{
 //以某种方式对地图进行排序并返回列表
} 
} 
  

解决方案

这不是 T 和<$ c的问题$ c> S ，但是使用 Comparable 和 Double 。

导致错误的原因是 Map< T，Double> 不是 Map< T，Comparable> 。

您必须扩大第二个类型参数的范围。例如：

  public static< T，S扩展了Comparable< S>>列表与LT; T>函数（Map< T，S> map）{
 //执行
} 
  

<然后，您可以使用以下方法调用该方法：

  Map< S，Double> map = new HashMap< S，Double>（）; 
函数（地图）; 
  

I have a static function with the following signature for a generic type T

public static<T> List<T> sortMap(Map<T, Comparable> map)

which should return a list of map keys with some property.

Now I want to pass a generic HashMap of type S

Map<S,Double> map

in calling the static function within a generic class, which has the map as a member variable.

I have listed a minimal code example below.

However, I get an error message (S and T are both T's but in different scopes of my code, i.e. T#1 = T, T#2= S):

  required: Map<T#1,Comparable>
  found: Map<T#2,Double>
  reason: cannot infer type-variable(s) T#1
  (argument mismatch; Map<T#2,Double> cannot be converted to Map<T#1,Comparable>)

How can resolve this issue? I am surprised that Java does not allow inferring a generic type from a generic type. What structure in Java can one use to work with that kind of more abstract code reasoning?

Code:

public class ExampleClass<T> {
    Map<T, Double> map;
    public ExampleClass () {
        this.map = new HashMap();
    }
    //the following line produces the mentioned error
    List<T> sortedMapKeys = UtilityMethods.sortMap(map);
}

public class UtilityMethods {
     public static<T> List<T> sortMap(Map<T, Comparable> map) {
        // sort the map in some way and return the list
     }
}

解决方案

It's not the problem with the T and S, but with the Comparable and Double.

The reason for the error is that a Map<T, Double> is not a Map<T, Comparable>.

You'll have to widen a bit the scope of the second type-parameter. Something like:

public static <T, S extends Comparable<S>> List<T> function(Map<T, S> map) {
    //implementation
}

Then, you'll be able to invoke the method with:

Map<S, Double> map = new HashMap<S, Double>();
function(map);

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