java 8选项列表收集到列表编译错误
我很难理解两者之间的区别java 8选项列表收集到列表编译错误,java,java-8,Java,Java 8,我很难理解两者之间的区别 Stream<Optional<Integer>> optionalStream = Stream.of( Optional.of(1), Optional.empty(), Optional.of(5)); List<Optional<Integer>> optionalList = optionalStream.colle
Stream<Optional<Integer>> optionalStream = Stream.of(
Optional.of(1),
Optional.empty(),
Optional.of(5));
List<Optional<Integer>> optionalList = optionalStream.collect(Collectors.toList());
Stream optionalStream=Stream.of(
(1)中的可选项,
可选。空(),
第(5)款的任择规定;
List optionalList=optionalStream.collect(Collectors.toList());
这很好用,而且:
List<Optional<Integer>> optionalList1 = Stream.of(
Optional.of(1),
Optional.empty(),
Optional.of(5)).collect(Collectors.toList());
List optionalList1=Stream.of(
(1)中的可选项,
可选。空(),
可选.of(5)).collect(collector.toList());
我在哪里收到错误
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>
Error:(138,40)java:不兼容类型:推理变量T的边界不兼容
等式约束:java.util.Optional
下限:java.util.OptionalStream.of(…)
或Optional.empty()
是通用方法。如果不提供类型参数,则将对其进行推断。对于Optional.empty()
您将得到Optional
因此Stream.of(Optional.of(1)、Optional.empty()、Optional.of(5))
将导致Stream我已经减少了一些示例,并尝试使用-xdwerbosesolution=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());
}
}
(首先提到的是“解析方法collect
)
没有约束它的目标类型
;实例化的签名显示它是一个流
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)
...
如果您添加一个提示(例如,Stream.of(/*etc*/
),第二个是否有效?是的!为什么会发生这种情况?好吧,您只是给类型推断算法更多的约束。实际上,更容易添加的是Optional.empty()
。感觉不应该这样做,但在当前的实现中就是这样。”对于可选的.empty()
您将得到可选的“但在第一种情况下,你不能这样做,否则它就不起作用。这个答案修复了症状,但没有解释差异。OP代码中的第一种和第二种情况都包含可选的.empty()
。如果,正如你所说,这是可选的类型,那么第一种情况也不会起作用。(顺便说一句,我不是下一个选民)。
...
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)
...
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)
...