Dfyjkt

I have below Scala code.

This function defines type parameter type B which is subclass of A. It converts java.util.List[A] into java.util.List[B].

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)

When I compile this program I get below error.

 <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())

What is wrong with my program. How can I resolve this error

It looks like type erasure happened (from Animal to ?0) when using java.util.stream.Stream.map, regardless B or concrete type passed in, possibly due to incompatibility between Scala type inference and Java type inference.

When calling Java method and you still want generic type inference, you need to pass the generic type specifically:

def convertLowerBound[ B <: Animal] (a: JList[Animal]): JList[B] = a.stream().map[B](a => a.asInstanceOf[B]).collect(Collectors.toList[B]())

Then you can do your operation successfully:

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]

On the other hand, your conversion isn't really useful, because during runtime, all the generic type will be erased so List[A] or List[B] will be the same after compiling (they will become List of Object. You can take a look at the compiled bytecode). You can simply do direct casting on your List instead of on each element:

def convertLowerBound[B <: Animal : TypeTag] (a: JList[Animal]) = a.asInstanceOf[JList[B]]

Then you can do your use case successfully:

scala> a.add(new Cat())
res16: Boolean = true

scala> convertLowerBound[Cat](a)
res17: java.util.List[Cat] = [Cat@6325af19]

However when there is a type mismatch, e.g. you add a Dog in the List and try to cast it as a List of Cat, an error will occur, only when you try to access the element:

scala> a.add(new Dog())
res19: Boolean = true

scala> convertLowerBound[Cat](a)
res20: java.util.List[Cat] = [Cat@6325af19, Dog@6ff6743f]

scala> convertLowerBound[Cat](a).get(1)
java.lang.ClassCastException: Dog cannot be cast to Cat
 ... 28 elided