R – Converting Enumeration to Iterator

enumerationgenericsiteratorscalatype-inference

I have the following implicit conversion for java.util.Enumerations

   implicit def enumerationIterator[A](e : Enumeration[A]) : Iterator[A] = {
     new Iterator[A] {
        def hasNext = e.hasMoreElements
        def next = e.nextElement
        def remove = throw new UnsupportedOperationException()
     }
   }

Unfortunately it does not work for ZipFile.entries because it returns an Enumeration<? extends ZipEntry> (see related question) and Scalac keeps telling me 

type mismatch; found : java.util.Iterator[?0] 
   where type ?0 <: java.util.zip.ZipEntry 
   required: Iterator[?]

I can't figure out how to make the conversation work in sth. like

List.fromIterator(new ZipFile(z).entries))

Best Solution

List.fromIterator expects a scala.Iterator but your implicit is returning a java.util.Iterator.

This works

import java.util.Enumeration

implicit def enum2Iterator[A](e : Enumeration[A]) = new Iterator[A] {
  def next = e.nextElement
  def hasNext = e.hasMoreElements
}

import java.util.zip.{ZipFile, ZipEntry}
val l = List.fromIterator(new ZipFile(null:java.io.File).entries)

Adding one import at the top prevents compilation 

import java.util.Iterator

There's been some discussion about unifying Scala and Java in 2.8 by just using java.util.Iterator. On the downside, Java's Iterator has a remove method which makes no sense for Scala's immutable collections. UnsupportedOperationException? Blech! On the plus side that makes stuff like this error go away.

Edit: I've added a Trac issue that the error message would have been clearer had it said "required: scala.Iterator[?]" https://lampsvn.epfl.ch/trac/scala/ticket/2102

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