使用JavaConversions在Java和Scala集合之间进行隐式转换

我使用通用函数合并了scala Set的scalaMap

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)

}

当存在相同的键冲突时,可以处理这种情况。但是,我想使用Scala

Code中的Java集合来实现。我研究了一下,发现了JavaConversions。我导入并编写了这个

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)

}

但是,它说类型不匹配

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)

^

是不是JavaConversions用来隐式转换util.HashMapmutable.Map?我在这里想念什么?

回答:

他们说尝试使用JavaConverters,因为不推荐使用JavaConversions。

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]

也许要说明为什么不推荐使用它:

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for理解会产生一对对。

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,推论既无法完成转换又无法找出操作类型。

有时将表达式分开会有助于推理,但不能在此进行。

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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