无法在 IntStream 上应用 Collectors.groupingBy

Question

此代码可以与其他对象配合使用，但不能与 int 配合使用，即使这些对象会自动装箱为 Integer。我无法理解错误消息。

jshell> String[] names = new String[]{"apple", "apple", "banana"};
names ==> String[3] { "apple", "apple", "banana" }

jshell> Map<String, Integer> stringMap = Arrays.stream(names).collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(e -> 1)));
stringMap ==> {banana=1, apple=2}

jshell> int[] nums = new int[]{1,2,3,3,4};
nums ==> int[5] { 1, 2, 3, 3, 4 }

jshell> Map<Integer, Integer> numsMap = Arrays.stream(nums).collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(e -> 1)));
|  Error:
|  method collect in interface java.util.stream.IntStream cannot be applied to given types;
|    required: java.util.function.Supplier<R>,java.util.function.ObjIntConsumer<R>,java.util.function.BiConsumer<R,R>
|    found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.Map<java.lang.Object,java.lang.Integer>>
|    reason: cannot infer type-variable(s) R
|      (actual and formal argument lists differ in length)
|  Map<Integer, Integer> numsMap = Arrays.stream(nums).collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(e -> 1)));
|                                  ^-------------------------^

Answer 1

这里没有自动装箱

正如 Pshemo 所评论的，

IntStream

不是 auto-box。您仅在

int

中使用

IntStream

原语。

如果您只需插入对

IntStream#boxed

的调用，您的代码就可以工作。

// Integer
int[] ints = new int[] { 1 , 2 , 3 , 3 , 4 };
Map < Integer, Integer > integersMap =
        Arrays
                .stream ( ints )
                .boxed ()
                .collect (
                        Collectors.groupingBy (
                                Function.identity ( ) ,
                                Collectors.summingInt ( e -> 1 )
                        )
                );
System.out.println ( "integersMap = " + integersMap );

stringMap = {香蕉=1，苹果=2}

并且，作为参考，

String

代码。

// String
String[] names = new String[] { "apple" , "apple" , "banana" };
Map < String, Integer > stringMap =
        Arrays
                .stream ( names )
                .collect (
                        Collectors.groupingBy (
                                Function.identity ( ) ,
                                Collectors.summingInt ( e -> 1 )
                        )
                );
System.out.println ( "stringMap = " + stringMap );

整数映射 = {1=1, 2=1, 3=2, 4=1}