java - “错误：不兼容的类型：推理变量 R 具有不兼容的边界”当 flatMap 单行流时

Question

我有一个自定义类Custom。

public class Custom {

  private Long id;

  List<Long> ids;

  // getters and setters
}

现在我有List<Custom>对象了。我想转换List<Custom>成List<Long>. 我已经编写了如下代码，并且工作正常。

    List<Custom> customs = Collections.emptyList();
    Stream<Long> streamL = customs.stream().flatMap(x -> x.getIds().stream());
    List<Long> customIds2 = streamL.collect(Collectors.toList());
    Set<Long> customIds3 = streamL.collect(Collectors.toSet());

现在我将 line2 和 line3 组合成一行，如下所示。

    List<Long> customIds = customs.stream().flatMap(x -> x.getIds().stream()).collect(Collectors.toSet());

现在，这段代码没有编译，我得到以下错误 -

    error: incompatible types: inference variable R has incompatible bounds
            List<Long> customIds = customs.stream().flatMap(x -> x.getIds().stream()).collect(Collectors.toSet());
                                                                                            ^
        equality constraints: Set<Long>
        upper bounds: List<Long>,Object
    where R,A,T are type-variables:
        R extends Object declared in method <R,A>collect(Collector<? super T,A,R>)
        A extends Object declared in method <R,A>collect(Collector<? super T,A,R>)
        T extends Object declared in interface Stream

我怎样才能转换List<Custom>成Set<Long>或List<Long>正确

标签： javajava-8java-stream

Answer 1

你可以这样做：

List<Custom> customs = Collections.emptyList();
Set<Long> customIdSet = customs.stream()
                               .flatMap(x -> x.getIds().stream())
                               .collect(Collectors.toSet()); // toSet and not toList

您收到编译器错误的原因是您使用了不正确Collector的返回 List 而不是 Set ，这是您将其分配给类型变量时的预期返回Set<Long>类型。