编译错误：在Scala中使用Java类的有界通配符问题

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英文:

Compilation error with bounded wildcards using Java classes in Scala

问题

在Java中，我们定义了一个名为ObservableCollection.java的类，如下所示：

  1. public class ObservableCollection<T> implements Collection<T> {
  3.    public SubscriptionHandle onElementAdded(Consumer<T> onAdded) {
  4.      // ... 
  5.    }
  6. }

还有一个返回ObservableCollection的AgentService.java：

  1. public interface AgentService {
  3.     ObservableCollection<? extends Agent> getAgents();
  5. }

现在，我想在Scala项目中使用这个ObservableCollection.java，代码如下：

  1. def test(service: AgentService): Unit = {
  2.   val onAdded: Consumer[_ <: Agent] = ???
  3.   service.getAgents.onElementAdded(onAdded)
  4. }

尝试这样做会导致以下编译错误：

  1. type mismatch;
  2.  found   : java.util.function.Consumer[_$1] where type _$1 <: com.xxxx.xx.xx.agent.Agent
  3.  required: java.util.function.Consumer[?0] where type ?0 <: com.xxxx.xx.xx.agent.Agent
  4.     service.getAgents.onElementAdded(onAdded)
  5.                                      ^
  6. one error found

这对我来说不太有意义。有没有方法可以让这个运行起来？

编辑： 使用Consumer[Agent]会导致以下错误：

  1. type mismatch;
  2.  found   : java.util.function.Consumer[com.xxxx.xx.xx.agent.Agent]
  3.  required: java.util.function.Consumer[?0] where type ?0 <: com.kuka.cc.si.agent.Agent
  4. Note: com.xxxx.xx.xx.agent.Agent >: ?0, but Java-defined trait Consumer is invariant in type T.
  5. You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
  6.     service.getAgents.onElementAdded(onAdded)
  7.                                      ^
  8. one error found
英文:

In Java, we have defined an ObservableCollection.java like this:

  1. public class ObservableCollection&lt;T&gt; implements Collection&lt;T&gt; {
  3.    public SubscriptionHandle onElementAdded(Consumer&lt;T&gt; onAdded) {
  4.      // ... 
  5.    }
  6. }

And an AgentService.java that returns an ObservableCollection:

  1. public interface AgentService {
  3.     ObservableCollection&lt;? extends Agent&gt; getAgents();
  5. }

Now, I am trying to use this ObservableCollection.java in a Scala project like this:

  1.   def test(service: AgentService): Unit = {
  2.     val onAdded: Consumer[_ &lt;: Agent] = ???
  3.     service.getAgents.onElementAdded(onAdded)
  4.   }

Trying this results in the following compilation error:

  1. type mismatch;
  2.  found   : java.util.function.Consumer[_$1] where type _$1 &lt;: com.xxxx.xx.xx.agent.Agent
  3.  required: java.util.function.Consumer[?0] where type ?0 &lt;: com.xxxx.xx.xx.agent.Agent
  4.     service.getAgents.onElementAdded(onAdded)
  5.                                      ^
  6. one error found

This does not make much sense to me. Is there a way I can get this running?

Edit: Using a Cosumer[Agent] results in the following error:

  1. type mismatch;
  2.  found   : java.util.function.Consumer[com.xxxx.xx.xx.agent.Agent]
  3.  required: java.util.function.Consumer[?0] where type ?0 &lt;: com.kuka.cc.si.agent.Agent
  4. Note: com.xxxx.xx.xx.agent.Agent &gt;: ?0, but Java-defined trait Consumer is invariant in type T.
  5. You may wish to investigate a wildcard type such as `_ &gt;: ?0`. (SLS 3.2.10)
  6.     service.getAgents.onElementAdded(onAdded)
  7.                                      ^
  8. one error found

答案1

得分: 2

以下是翻译好的内容：

首先是关于 Scala-Java 互操作性的内容：

  1. 不在于 Scala-Java 互操作性。以下 Scala 代码也无法编译：
  3. import java.util.function.Consumer
  4. import java.util
  6. trait Agent
  7. trait SubscriptionHandle
  8. trait AgentService {
  9.   def getAgents: ObservableCollection[_ <: Agent]
  10. }
  11. trait ObservableCollection[T] extends util.Collection[T] {
  12.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  13. }
  15. def test(service: AgentService): Unit = {
  16.   val onAdded: Consumer[_ <: Agent] = ???
  17.   val agents: ObservableCollection[_ <: Agent] = service.getAgents
  18.   agents.onElementAdded(onAdded)
  19. }
  20. // 类型不匹配；
  21. // 找到：java.util.function.Consumer[_$2]，其中类型 _$2 <: App.Agent
  22. // 需要：java.util.function.Consumer[_$3]

你错误地使用了存在类型（通配符泛型）。以下代码无法编译：

  1. trait X[T]
  2. trait Y[T] {
  3.   def foo(x: X[T]) = ???
  4. }
  5. val x: X[_] = ???
  6. val y: Y[_] = ???
  7. y.foo(x) // 无法编译

x 和 y 都具有存在类型，但 foo 接受类型为 X[T] 的 x，其中 T 必须与 y 的类型中的 T 相同，即 Y[T]，因此无法保证 T 相同。

修复编译错误的一种方法是为 AgentService 添加泛型：

  1. trait Agent
  2. trait SubscriptionHandle
  3. trait AgentService[T <: Agent] {
  4.   def getAgents: ObservableCollection[T]
  5. }
  6. trait ObservableCollection[T] extends util.Collection[T] {
  7.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  8. }
  10. def test[T <: Agent](service: AgentService[T]): Unit = {
  11.   val onAdded: Consumer[T] = ???
  12.   val agents: ObservableCollection[T] = service.getAgents
  13.   agents.onElementAdded(onAdded)
  14. }

或者将泛型添加到其方法中：

  1. trait Agent
  2. trait SubscriptionHandle
  3. trait AgentService {
  4.   def getAgents[T <: Agent]: ObservableCollection[T]
  5. }
  6. trait ObservableCollection[T] extends util.Collection[T] {
  7.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  8. }
  10. def test[T <: Agent](service: AgentService): Unit = {
  11.   val onAdded: Consumer[T] = ???
  12.   val agents: ObservableCollection[T] = service.getAgents
  13.   agents.onElementAdded(onAdded)
  14. }
英文:

The thing is not in Scala-Java interop. The following Scala code doesn't compile either

  1. import java.util.function.Consumer
  2. import java.util
  4. trait Agent
  5. trait SubscriptionHandle
  6. trait AgentService {
  7.   def getAgents: ObservableCollection[_ &lt;: Agent]
  8. }
  9. trait ObservableCollection[T] extends util.Collection[T] {
  10.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  11. }
  13. def test(service: AgentService): Unit = {
  14.   val onAdded: Consumer[_ &lt;: Agent] = ???
  15.   val agents: ObservableCollection[_ &lt;: Agent] = service.getAgents
  16.   agents.onElementAdded(onAdded)
  17. //                      ^^^^^^^
  18. }
  20. //type mismatch;
  21. // found   : java.util.function.Consumer[_$2] where type _$2 &lt;: App.Agent
  22. // required: java.util.function.Consumer[_$3]

You misuse existential types (wildcard generics). The following code can't compile

  1. trait X[T]
  2. trait Y[T] {
  3.   def foo(x: X[T]) = ???
  4. }
  5. val x: X[_] = ???
  6. val y: Y[_] = ???
  7. y.foo(x) // doesn&#39;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, i.e. Y[T], so you can't guarantee that T are the same.

One way to fix compilation is to add generics to AgentService

  1. trait Agent
  2. trait SubscriptionHandle
  3. trait AgentService[T &lt;: Agent] {
  4.   def getAgents: ObservableCollection[T]
  5. }
  6. trait ObservableCollection[T] extends util.Collection[T] {
  7.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  8. }
  10. def test[T &lt;: Agent](service: AgentService[T]): Unit = {
  11.   val onAdded: Consumer[T] = ???
  12.   val agents: ObservableCollection[T] = service.getAgents
  13.   agents.onElementAdded(onAdded)
  14. }

or to its method

  1. trait Agent
  2. trait SubscriptionHandle
  3. trait AgentService {
  4.   def getAgents[T &lt;: Agent]: ObservableCollection[T]
  5. }
  6. trait ObservableCollection[T] extends util.Collection[T] {
  7.   def onElementAdded(onAdded: Consumer[T]): SubscriptionHandle
  8. }
  10. def test[T &lt;: Agent](service: AgentService): Unit = {
  11.   val onAdded: Consumer[T] = ???
  12.   val agents: ObservableCollection[T] = service.getAgents
  13.   agents.onElementAdded(onAdded)
  14. }

huangapple
  • 本文由 发表于 2020年8月24日 20:47:58
  • 转载请务必保留本文链接：https://go.coder-hub.com/63561372.html
  • bounded-wildcard
  • java
  • scala
  • scala-java-interop
  • type-conversion
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