[英]Compilation error with bounded wildcards using Java classes in Scala
In Java, we have defined an ObservableCollection.java
like this:在 Java 中,我们定义了一个
ObservableCollection.java
如下所示:
public class ObservableCollection<T> implements Collection<T> {
public SubscriptionHandle onElementAdded(Consumer<T> onAdded) {
// ...
}
}
And an AgentService.java
that returns an ObservableCollection:和
AgentService.java
返回一个ObservableCollection:
public interface AgentService {
ObservableCollection<? extends Agent> getAgents();
}
Now, I am trying to use this ObservableCollection.java
in a Scala project like this:现在,我试图在像这样的Scala项目中使用这个
ObservableCollection.java
:
def test(service: AgentService): Unit = {
val onAdded: Consumer[_ <: Agent] = ???
service.getAgents.onElementAdded(onAdded)
}
Trying this results in the following compilation error:尝试这样做会导致以下编译错误:
type mismatch;
found : java.util.function.Consumer[_$1] where type _$1 <: com.xxxx.xx.xx.agent.Agent
required: java.util.function.Consumer[?0] where type ?0 <: com.xxxx.xx.xx.agent.Agent
service.getAgents.onElementAdded(onAdded)
^
one error found
This does not make much sense to me.这对我来说没有多大意义。 Is there a way I can get this running?
有什么办法可以让它运行吗?
Edit: Using a Cosumer[Agent]
results in the following error:编辑:使用
Cosumer[Agent]
导致以下错误:
type mismatch;
found : java.util.function.Consumer[com.xxxx.xx.xx.agent.Agent]
required: java.util.function.Consumer[?0] where type ?0 <: com.kuka.cc.si.agent.Agent
Note: com.xxxx.xx.xx.agent.Agent >: ?0, but Java-defined trait Consumer is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
service.getAgents.onElementAdded(onAdded)
^
one error found
The thing is not in Scala-Java interop.事情不在 Scala-Java 互操作中。 The following Scala code doesn't compile either
以下 Scala 代码也无法编译
import java.util.function.Consumer
import java.util
trait Agent
trait SubscriptionHandle
trait AgentService {
def getAgents: ObservableCollection[_ <: Agent]
}
trait ObservableCollection[T] extends util.Collection[T] {
def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}
def test(service: AgentService): Unit = {
val onAdded: Consumer[_ <: Agent] = ???
val agents: ObservableCollection[_ <: Agent] = service.getAgents
agents.onElementAdded(onAdded)
// ^^^^^^^
}
//type mismatch;
// found : java.util.function.Consumer[_$2] where type _$2 <: App.Agent
// required: java.util.function.Consumer[_$3]
You misuse existential types (wildcard generics).您滥用了存在类型(通配符泛型)。 The following code can't compile
以下代码无法编译
trait X[T]
trait Y[T] {
def foo(x: X[T]) = ???
}
val x: X[_] = ???
val y: Y[_] = ???
y.foo(x) // doesn't compile
Both x
and y
have existential types but foo
accepts x
of type X[T]
, where T
must be the same as T
in the type of y
, ie Y[T]
, so you can't guarantee that T
are the same.这两个
x
和y
有存在的类型,但foo
接受x
型的X[T]
其中T
必须是一样T
的类型y
,即Y[T]
所以你不能保证T
是相同的。
One way to fix compilation is to add generics to AgentService
修复编译的一种方法是向
AgentService
添加泛型
trait Agent
trait SubscriptionHandle
trait AgentService[T <: Agent] {
def getAgents: ObservableCollection[T]
}
trait ObservableCollection[T] extends util.Collection[T] {
def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}
def test[T <: Agent](service: AgentService[T]): Unit = {
val onAdded: Consumer[T] = ???
val agents: ObservableCollection[T] = service.getAgents
agents.onElementAdded(onAdded)
}
or to its method或者到它的方法
trait Agent
trait SubscriptionHandle
trait AgentService {
def getAgents[T <: Agent]: ObservableCollection[T]
}
trait ObservableCollection[T] extends util.Collection[T] {
def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
}
def test[T <: Agent](service: AgentService): Unit = {
val onAdded: Consumer[T] = ???
val agents: ObservableCollection[T] = service.getAgents
agents.onElementAdded(onAdded)
}
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