Scala error: type mismatch; found : java.util.List[?0] required: java.util.List[B]









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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










share|improve this question



























    up vote
    1
    down vote

    favorite












    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










    share|improve this question

























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      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










      share|improve this question















      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







      scala






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 9 at 19:04

























      asked Nov 9 at 18:18









      Rajkumar Natarajan

      1,1171237




      1,1171237






















          1 Answer
          1






          active

          oldest

          votes

















          up vote
          3
          down vote



          accepted










          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





          share|improve this answer






















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            1 Answer
            1






            active

            oldest

            votes








            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes








            up vote
            3
            down vote



            accepted










            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





            share|improve this answer


























              up vote
              3
              down vote



              accepted










              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





              share|improve this answer
























                up vote
                3
                down vote



                accepted







                up vote
                3
                down vote



                accepted






                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





                share|improve this answer














                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






                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited Nov 9 at 20:15

























                answered Nov 9 at 20:05









                texasbruce

                8,27874193




                8,27874193



























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