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

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) }     | }