java - "error: incompatible types: inference variable R has incompatible bounds"单行flatMap流

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。我想转换 List<Custom>进入List<Long> .我编写了如下代码，它运行良好。

    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，我遇到了错误 -

    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

如何转换 List<Custom>进入Set<Long>或 List<Long>正确

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

你得到编译器错误的原因是你使用了不正确的 Collector它返回一个 List 而不是 Set ，这是您将其分配给 Set<Long> 的变量时预期的返回类型类型。