注意 JavaConverters._   不要忘了 ._ !!!
import scala.collection.JavaConverters._

scala2.12开始，废除 scala.collection.JavaConversions ，
改用 scala.collection.JavaConverters


val list = new java.util.ArrayList[Int]()
list: java.util.ArrayList[Int] = []

list.add(1)

list.asScala
res17: scala.collection.mutable.Buffer[Int] = Buffer(1)

val a = List(1,2,3)
a: List[Int] = List(1, 2, 3)

a.asJava
res18: java.util.List[Int] = [1, 2, 3]

val seq = Seq(1,2,3)
seq: Seq[Int] = List(1, 2, 3)

seq.asJava
res19: java.util.List[Int] = [1, 2, 3]




 val list = List(1,2)
list: List[Int] = List(1, 2)

scala> list.asJava
res0: java.util.List[Int] = [1, 2]

scala> val javaList = list.asJava
javaList: java.util.List[Int] = [1, 2]

scala> javaList.asScala
res1: scala.collection.mutable.Buffer[Int] = Buffer(1, 2)

scala> javaList.asScala.toList
res2: List[Int] = List(1, 2)
 javaList.asScala.toList(0)
res5: Int = 1



 javaList.asScala.toSeq
res3: Seq[Int] = Buffer(1, 2)

javaList.asScala.toSeq(0)
res4: Int = 1