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.HashMap
为mutable.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 kvmergeMaps: [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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