Scala error: type mismatch; found : java.util.List[?0] required: java.util.List[B]
Problem:
Due to the incompatibility between Scala type inference and Java type inference;
import java.util
import java.util.stream.Collectors
class Animal
class Dog extends Animal
class Cat extends Animal
object ObjectConversions extends App {
import java.util.{List => JList}
implicit def convertLowerBound[ B <: Animal] (a: JList[Animal]): JList[B] = a.stream().map(a => a.asInstanceOf[B]).collect(Collectors.toList())
val a= new util.ArrayList[Animal]()
a.add(new Cat)
convertLowerBound[Cat](a)
}
Solution:
When calling Java methods and still want to infer generic types, you need to pass generic types specifically
#Or the implicit conversion statement is not imported
import scala.collection.JavaConversions._
def convertLowerBound[ B <: Animal] (a: JList[Animal]): JList[B] = a.stream().map[B](a => a.asInstanceOf[B]).collect(Collectors.toList[B]())
#or
def convertLowerBound[B <: Animal : TypeTag] (a: JList[Animal]) = a.asInstanceOf[JList[B]]
scala> def convertLowerBound[ B <: Animal] (a: JList[Animal]): JList[B] = a.stream().map[B](a => a.asInstanceOf[B]).collect(Collectors.toList[B]())
convertLowerBound: [B <: Animal](a: java.util.List[Animal])java.util.List[B]
scala> convertLowerBound[Cat](a)
res30: java.util.List[Cat] = [Cat@6325af19, Dog@6ff6743f]
scala> a.add(new Cat())
res16: Boolean = true
scala> convertLowerBound[Cat](a)
res17: java.util.List[Cat] = [Cat@6325af19]
scala> a.add(new Dog())
res19: Boolean = true
scala> convertLowerBound[Cat](a)
res20: java.util.List[Cat] = [Cat@6325af19, Dog@6ff6743f]
完整错误：
<console>:15: error: type mismatch;
found : java.util.List[?0]
required: java.util.List[B]
Note: ?0 >: B, but Java-defined trait List is invariant in type E.
You may wish to investigate a wildcard type such as `_ >: B`. (SLS 3.2.10)
implicit def convertLowerBound[ B <: Animal] (a: JList[Animal]): JList[B] = a.stream().map(a => a.asInstanceOf[B]).collect(Collectors.toList())
