编译器和运行时之间的 Scala 类型不一致？

Question

从 java.util.ArrayList 转换为 scala.collection.immutable.List，2.10 编译器和运行时似乎不一致，关于类型val 发出:

import org.ahocorasick.trie._
import scala.collection.JavaConverters._    // convert Java colllections to Scala ones

object wierd {

  val trie = new Trie

  def trieInit(patterns: List[String]) {
    trie.onlyWholeWords();
    for (pattern <- patterns)
      trie.addKeyword(pattern)
  }

  def patternTest(text : String) : List[String] = 
  {
    val emitsJ = trie.parseText(text)
    val emits = emitsJ.asScala map (i => i.getKeyword)

    println(s"converted from ${emitsJ.getClass} to ${emits.getClass}")

    //return(emits)
    return (List.empty[String])
  }

  trieInit(List("hello"))
  patternTest("hello")
}

产量:

从类 java.util.ArrayList 转换为类 scala.collection.immutable.$colon$colon

现在通过仅更改 return 行更改为返回实际值 -

import org.ahocorasick.trie._
import scala.collection.JavaConverters._    // convert Java colllections to Scala ones

object wierd {

  val trie = new Trie

  def trieInit(patterns: List[String]) {
    trie.onlyWholeWords();
    for (pattern <- patterns)
      trie.addKeyword(pattern)
  }

  def patternTest(text : String) : List[String] = 
  {
    val emitsJ = trie.parseText(text)
    val emits = emitsJ.asScala map (i => i.getKeyword)

    println(s"converted from ${emitsJ.getClass} to ${emits.getClass}")

    return(emits)
    //return (List.empty[String])
  }

  trieInit(List("hello"))
  patternTest("hello")
}

产生编译错误:

[error] reproduce.scala:23: type mismatch;
[error]  found   : Iterable[String]
[error]  required: List[String]
[error]     return(emits)
[error]            ^
[error] one error found
[error] (compile:compile) Compilation failed

对此的简单解释是什么？我怎样才能更好地处理转化？

Answer 1

JavaConverters 转换为最接近它已拉皮条的 Java 集合的 Scala 集合。您仍然需要调用 toList 将其进一步转换为您想要的集合:

val emits = emitsJ.asScala.toList map (i => i.getKeyword)

参见相关:What is the difference between JavaConverters and JavaConversions in Scala?