[英]Implicit conversion between java and scala collections using JavaConversions
I merged a scala Set
of scala Map
s using a generic function 我使用通用函数合并了一个scala Set
of scala Map
。
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)
}
This handles the case when there is a clash of same keys. 当相同的键发生冲突时,可以处理这种情况。 However, I wanted to do it with Java collections in Scala Code. 但是,我想使用Scala Code中的Java集合来实现。 I researched a bit and came across JavaConversions
. 我研究了一下,发现了JavaConversions
。 I imported it and wrote this 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)
}
However, it says there is a type mismatch 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)
^
Is not JavaConversions
used to implicitly convert util.HashMap
to mutable.Map
? JavaConversions
不用于将util.HashMap
隐式转换为mutable.Map
吗? What am I missing here? 我在这里想念什么?
Would a JavaConverter
do what you want? JavaConverter
做什么吗?
scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._
scala> val x = (new java.util.HashMap[Int,Int]).asScala
x: scala.collection.mutable.Map[Int,Int] = Map()
They say to try JavaConverters, as JavaConversions is deprecated. 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]
Perhaps to show why it's deprecated: 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)) {
^
Noting that the for comprehension yields a set of pairs. 注意for理解会产生一组对。
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)]
Apparently inference fails to both do the conversion and then figure out the op types. 显然,推理无法完成转换并找出运算类型。
Sometimes breaking apart the expression assists inference, but not here. 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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