java 8选项列表收集列表编译错误

时间:2017-10-08 21:37:23

标签: java java-8

我无法理解

之间的区别
Stream<Optional<Integer>> optionalStream = Stream.of(
                Optional.of(1),
                Optional.empty(),
                Optional.of(5));

List<Optional<Integer>> optionalList = optionalStream.collect(Collectors.toList());

哪种方法很好,并且:

List<Optional<Integer>> optionalList1 = Stream.of(
                Optional.of(1),
                Optional.empty(),
                Optional.of(5)).collect(Collectors.toList());

我收到错误的地方

Error:(138, 40) java: incompatible types: inference variable T has incompatible bounds
    equality constraints: java.util.Optional<java.lang.Integer>
    lower bounds: java.util.Optional<? extends java.lang.Object>

2 个答案:

答案 0 :(得分:4)

我稍微减少了一些示例,并尝试使用-XDverboseResolution=all编译以输出有关类型推断的信息:

final class One {
  void one() {
    Stream<Optional<Integer>> optionalStream = Stream.of(Optional.empty());
    List<Optional<Integer>> optionalList = optionalStream.collect(Collectors.toList());
  }
}

final class Two {
  void two() {
    List<Optional<Integer>> optionalList1 =
        Stream.of(Optional.empty()).collect(Collectors.toList());
  }
}

对于Two,看起来Stream.of的延迟实例化甚至在查看后续collect之前就完成了:

...
Two.java:9: Note: Deferred instantiation of method <T>of(T)
        Stream.of(Optional.empty()).collect(Collectors.toList());
                 ^
  instantiated signature: (Optional<Object>)Stream<Optional<Object>>
  target-type: <none>
  where T is a type-variable:
    T extends Object declared in method <T>of(T)
Two.java:9: Note: resolving method collect in type Stream to candidate 0
        Stream.of(Optional.empty()).collect(Collectors.toList());
...

(“解析方法collect是第一次提及collect

没有target-type来限制它;实例化的签名显示它是Stream<Optional<Object>>

如果查看One的相应输出:

...
One.java:8: Note: Deferred instantiation of method <T>of(T)
    Stream<Optional<Integer>> optionalStream = Stream.of(Optional.empty());
                                                        ^
  instantiated signature: (Optional<Integer>)Stream<Optional<Integer>>
  target-type: Stream<Optional<Integer>>
  where T is a type-variable:
    T extends Object declared in method <T>of(T)
...

它是正确的,因为它知道目标类型。

我不能确切地说为什么延迟实例化在Two的此时发生,因为我对应用类型推断的方式不太熟悉。

认为是因为Stream.of的调用不被视为多元表达,但我无法真正说服自己为什么(请参阅编辑历史记录以了解一些不连贯的翻译)。 / p>

我建议的修正方法是对Optional.empty()应用类型提示,即Optional.<Integer>empty()。这具有在推理中更早获得Optional的实际类型的效果,因此在延迟实例化时已知事件,尽管目标类型仍未知:

final class Three {
  void three() {
    List<Optional<Integer>> optionalList1 =
        Stream.of(Optional.<Integer>empty()).collect(Collectors.toList());
  }
}

...
Three.java:9: Note: resolving method of in type Stream to candidate 1
        Stream.of(Optional.<Integer>empty()).collect(Collectors.toList());
              ^
  phase: BASIC
  with actuals: Optional<Integer>
  with type-args: no arguments
  candidates:
      #0 not applicable method found: <T#1>of(T#1...)
        (cannot infer type-variable(s) T#1
          (argument mismatch; Optional<Integer> cannot be converted to T#1[]))
      #1 applicable method found: <T#2>of(T#2)
        (partially instantiated to: (Optional<Integer>)Stream<Optional<Integer>>)
  where T#1,T#2 are type-variables:
    T#1 extends Object declared in method <T#1>of(T#1...)
    T#2 extends Object declared in method <T#2>of(T#2)
Three.java:9: Note: Deferred instantiation of method <T>of(T)
        Stream.of(Optional.<Integer>empty()).collect(Collectors.toList());
                 ^
  instantiated signature: (Optional<Integer>)Stream<Optional<Integer>>
  target-type: <none>
  where T is a type-variable:
    T extends Object declared in method <T>of(T)
...

答案 1 :(得分:3)

Stream.of(...)Optional.empty()是通用方法。如果您不提供type参数,则会推断出它。对于Optional.empty(),您将获得Optional<Object>,因此Stream.of(Optional.of(1), Optional.empty(), Optional.of(5))会产生Stream<Optional<? extends Object>>

您可以通过在Optional.<Integer>empty()Stream.<Optional<Integer>>of(...)中提供类型参数来解决此问题。我更喜欢第一个。