Function::identity 在 Collectors.toMap 中不起作用

Question

我试图将 a 转换List<String>为 a Map<T, String>，映射的值是前一个 中包含的元素List<String>，键是该元素的某个属性String（例如， 的长度String，在这种情况下，T实际上是 an Integer）。

我首先尝试这样做。这是我上面提到的示例的实现。我希望 的长度String成为键，而其String本身成为值。我想使用该Function::identity函数来明确指定该值是其String本身。

import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class SOQ_ME_20220522_fail
{

   public static void main(String[] args)
   {
   
      final List<String> list = List.of("apple", "banana", "coconut");
      
      var result = 
         list.stream()
            .collect(
               Collectors.toMap(
                  each -> each.length(),
                  Function::identity
               )
            )
         ;

      System.out.println(result);
   
   }
   
}

但是，当我尝试编译它时，出现以下编译错误。

SOQ_ME_20220522_fail.java:15: error: no suitable method found for toMap((each)->ea[...]gth(),Function::identity)
               Collectors.toMap(
                         ^
    method Collectors.<T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>) is not applicable
      (cannot infer type-variable(s) T#1,K#1,U#1
        (argument mismatch; unexpected static method <T#2>identity() found in unbound lookup))
    method Collectors.<T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>) is not applicable
      (cannot infer type-variable(s) T#3,K#2,U#2
        (actual and formal argument lists differ in length))
    method Collectors.<T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>) is not applicable
      (cannot infer type-variable(s) T#4,K#3,U#3,M
        (actual and formal argument lists differ in length))
  where T#1,K#1,U#1,T#2,T#3,K#2,U#2,T#4,K#3,U#3,M are type-variables:
    T#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    K#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    U#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    T#2 extends Object declared in method <T#2>identity()
    T#3 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    K#2 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    U#2 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    T#4 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    K#3 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    U#3 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    M extends Map<K#3,U#3> declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
1 error

现在，这很容易解决 - 我只需要通过替换为 来牺牲一些Function::identity可读性(t -> t)。这样做，该类就可以正常编译和运行。

SOQ_ME_20220522_fail.java:15: error: no suitable method found for toMap((each)->ea[...]gth(),Function::identity)
               Collectors.toMap(
                         ^
    method Collectors.<T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>) is not applicable
      (cannot infer type-variable(s) T#1,K#1,U#1
        (argument mismatch; unexpected static method <T#2>identity() found in unbound lookup))
    method Collectors.<T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>) is not applicable
      (cannot infer type-variable(s) T#3,K#2,U#2
        (actual and formal argument lists differ in length))
    method Collectors.<T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>) is not applicable
      (cannot infer type-variable(s) T#4,K#3,U#3,M
        (actual and formal argument lists differ in length))
  where T#1,K#1,U#1,T#2,T#3,K#2,U#2,T#4,K#3,U#3,M are type-variables:
    T#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    K#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    U#1 extends Object declared in method <T#1,K#1,U#1>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>)
    T#2 extends Object declared in method <T#2>identity()
    T#3 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    K#2 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    U#2 extends Object declared in method <T#3,K#2,U#2>toMap(Function<? super T#3,? extends K#2>,Function<? super T#3,? extends U#2>,BinaryOperator<U#2>)
    T#4 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    K#3 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    U#3 extends Object declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
    M extends Map<K#3,U#3> declared in method <T#4,K#3,U#3,M>toMap(Function<? super T#4,? extends K#3>,Function<? super T#4,? extends U#3>,BinaryOperator<U#3>,Supplier<M>)
1 error

{5=apple, 6=banana, 7=coconut}

我的问题是 - 为什么我无法使用Function::identity来获取我需要的数据？查看 的文档Function::identity，我们可以看到这个静态接口方法应该返回 a，Function<T, T>该方法返回它接收的对象。如果我们查看源代码本身，我们可以看到它创建了与我创建的完全相同的 lambda。所以我很困惑为什么第一次尝试失败了。显然，我能够轻松地通过它，但我仍然想知道为什么第一次尝试失败了。

Answer 1

Function::identity用。。。来代替Function.identity()。

前者是方法引用，只有在您有Function对象流并且想要identity对每个对象调用方法时才有效。后者只是一个静态方法调用，它返回Function您作为值映射器参数传递的 a 。

两个注意事项：

1. lambda表达式each -> each.length()可以写为String::length（方法引用）。当然，如果您愿意，也可以将其保留为 lambda 表达式。
2. 如果多个元素可以映射到相同的键（例如，在您的示例中，如果多个字符串可以具有相同的长度），则考虑提供合并函数或使用Collectors#groupingBy(...).