无法引用Java 8 Stream非静态方法流 [英] Java 8 Stream non static method stream cannot be referenced

问题描述

我在IDEA中遇到流问题. 我有一个简单的测试:

I have problem with streams in IDEA. I have a simple test:

 @Test
    public void shouldFlattenAListOfList() throws Exception {
        List<String> flatten = PTestImpl.flatten(
                asList("a", asList("b", asList("c", "d")), "e"));
        assertThat(flatten, hasSize(5));
        assertThat(flatten, hasItems("a", "b", "c", "d", "e"));
    }

和静态方法

 public static <T> List<T> flatten(List<?> list){
        return list.stream()
                .flatMap(Collection::stream)
                .collect(Collectors.toList());;
    }

但是IDEA告诉我:

Error:(44, 26) java: invalid method reference
  non-static method stream() cannot be referenced from a static context

我该如何解决?

推荐答案

如果您不想对接收到的元素的类型做任何假设，则必须更改flatten方法主体及其返回类型.列表:

The flatten method body and its return type has to be changed, if you do not want to make any assumptions about the type of the elements in the received list:

public static List<?> flatten(List<?> list){
    return list.stream()
        .flatMap(e -> e instanceof List ? flatten((List) e).stream() : Stream.of(e))
        .collect(Collectors.toList());
}

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