使用JavaConversions在java和scala集合之间进行隐式转换 [英] Implicit conversion between java and scala collections using JavaConversions

问题描述

我使用通用函数合并了scala 设置 scala Map s

I merged a scala Set of scala Maps using a generic function

def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
(Map[A, B]() /: (for (m <- ms; kv <- m) yield kv))
{
  (a, kv) =>
  a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv)
}

这可以处理存在相同键冲突的情况。但是，我想在Scala Code中使用Java集合来实现它。我研究了一下，遇到了 JavaConversions 。我导入它并写了这个

This handles the case when there is a clash of same keys. However, I wanted to do it with Java collections in Scala Code. I researched a bit and came across JavaConversions. I imported it and wrote this

def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
(new util.HashMap[A, B] /: (for (m <- ms; kv <- m) yield kv))
{
  case (a, kv) =>
    a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv)
}

然而，它表示类型不匹配

However, it says there is a type mismatch

Error:(67, 11) type mismatch;
found   : scala.collection.mutable.Map[A,B]
required: java.util.HashMap[A,B]
    a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv)
      ^

不是 JavaConversions 用于隐式转换 util.HashMap 到 mutable.Map ？我在这里缺少什么？

Is not JavaConversions used to implicitly convert util.HashMap to mutable.Map? What am I missing here?

推荐答案

他们说要尝试JavaConverters，因为JavaConversions已被弃用。

They say to try JavaConverters, as JavaConversions is deprecated.

scala> import collection.JavaConverters._
import collection.JavaConverters._

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
     | (new java.util.HashMap[A, B] /: (for (m <- ms; kv <- m) yield kv)) {
     | case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
<console>:16: error: value contains is not a member of java.util.HashMap[A,B]
       case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
                                  ^
<console>:16: error: java.util.HashMap[A,B] does not take parameters
       case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
                                                               ^
<console>:16: error: type mismatch;
 found   : (A, B)
 required: String
       case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
                                                                                    ^
<console>:15: error: type mismatch;
 found   : java.util.HashMap[A,B]
 required: Map[A,B]
       (new java.util.HashMap[A, B] /: (for (m <- ms; kv <- m) yield kv)) {
                                    ^

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
     | (new java.util.HashMap[A, B].asScala /: (for (m <- ms; kv <- m) yield kv)) {
     | case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
<console>:15: error: type mismatch;
 found   : scala.collection.mutable.Map[A,B]
 required: scala.collection.immutable.Map[A,B]
       (new java.util.HashMap[A, B].asScala /: (for (m <- ms; kv <- m) yield kv)) {
                                            ^

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
     | (new java.util.HashMap[A, B].asScala.toMap /: (for (m <- ms; kv <- m) yield kv)) {
     | case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
mergeMaps: [A, B](ms: Set[Map[A,B]])(f: (B, B) => B)Map[A,B]

也许是为了说明它被弃用的原因：

Perhaps to show why it's deprecated:

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] =
     | (new java.util.HashMap[A, B] /: (for (m <- ms; kv <- m) yield kv)) {
     | case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
<console>:19: error: type mismatch;
 found   : scala.collection.mutable.Map[A,B]
 required: java.util.HashMap[A,B]
       case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
                         ^
<console>:18: error: type mismatch;
 found   : java.util.HashMap[A,B]
 required: Map[A,B]
       (new java.util.HashMap[A, B] /: (for (m <- ms; kv <- m) yield kv)) {
                                    ^

注意到for comprehension产生一组对。

Noting that the for comprehension yields a set of pairs.

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B) = for (m <- ms; kv <- m) yield kv
mergeMaps: [A, B](ms: Set[Map[A,B]])(f: (B, B) => B)scala.collection.immutable.Set[(A, B)]

显然，推理无法进行转换，然后找出操作类型。

Apparently inference fails to both do the conversion and then figure out the op types.

有时会破坏表达式辅助推理，但不会在这里。

Sometimes breaking apart the expression assists inference, but not here.

scala> def mergeMaps[A, B](ms: Set[Map[A, B]])(f: (B, B) => B): Map[A, B] = {
     | val ss = for (m <- ms; kv <- m) yield kv
     | (new java.util.HashMap[A, B] /: ss) {
     | case (a, kv) => a + (if (a.contains(kv._1)) kv._1 -> f(a(kv._1), kv._2) else kv) }
     | }

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