从数组流式传输时，为什么我不能将整数映射到字符串?

2022-01-22 00:00:00 functional-programming java-8 java java-stream

此代码有效(取自 Javadoc):

This code works (taken in the Javadoc):

List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
String commaSeparatedNumbers = numbers.stream()
    .map(i -> i.toString())
    .collect(Collectors.joining(", "));

这个不能编译:

int[] numbers = {1, 2, 3, 4};
String commaSeparatedNumbers = Arrays.stream(numbers)
    .map((Integer i) -> i.toString())
    .collect(Collectors.joining(", "));

IDEA 告诉我lambda 表达式中的返回类型字符串不兼容".

IDEA tells me I have an "incompatible return type String in lambda expression".

为什么?以及如何解决这个问题?

Why ? And how to fix that ?

推荐答案

Arrays.stream(int[]) 创建一个IntStream，而不是一个Stream<整数>.所以你需要调用 mapToObj 在将 int 映射到对象时，而不仅仅是 map.

Arrays.stream(int[]) creates an IntStream, not a Stream<Integer>. So you need to call mapToObj instead of just map, when mapping an int to an object.

这应该可以按预期工作:

This should work as expected:

String commaSeparatedNumbers = Arrays.stream(numbers)
    .mapToObj(i -> ((Integer) i).toString()) //i is an int, not an Integer
    .collect(Collectors.joining(", "));

你也可以这样写:

String commaSeparatedNumbers = Arrays.stream(numbers)
    .mapToObj(Integer::toString)
    .collect(Collectors.joining(", "));

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