How to get Scala List from Java List?
JavaScalaJava Problem Overview
I have a Java API that returns a List like:
public List<?> getByXPath(String xpathExpr)
I am using the below scala code:
val lst = node.getByXPath(xpath)
Now if I try scala syntax sugar like:
lst.foreach{ node => ... }
it does not work. I get the error:
value foreach is not a member of java.util.List[?0]
It seems I need to convert Java List to Scala List. How to do that in above context?
Java Solutions
Solution 1 - Java
JavaConverters
instead. (comment by @Yaroslav)
EDIT: Note that this is deprecated since 2.12.0. Use Since Scala 2.8 this conversion is now built into the language using:
import scala.collection.JavaConversions._
...
lst.toList.foreach{ node => .... }
works. asScala
did not work
In 2.12.x use import scala.collection.JavaConverters._
In 2.13.x use import scala.jdk.CollectionConverters._
Solution 2 - Java
There's a handy Scala object just for this - scala.collection.JavaConverters
You can do the import and asScala
afterwards as follows:
import scala.collection.JavaConverters._
val lst = node.getByXPath(xpath).asScala
lst.foreach{ node => .... }
This should give you Scala's Buffer
representation allowing you to accomplish foreach
.
Solution 3 - Java
I was looking for an answer written in Java and surprisingly couldn't find any clean solutions here. After a while I was able to figure it out so I decided to add it here in case someone else is looking for the Java implementation (I guess it also works in Scala?):
JavaConversions.asScalaBuffer(myJavaList).toList()
Solution 4 - Java
If you have to convert a Java List<ClassA>
to a Scala List[ClassB]
, then you must do the following:
-
Add
import scala.collection.JavaConverters._
-
Use methods
asScala
,toList
and thenmap
List
javaList = ... var scalaList[ClassB] = javaList.asScala.toList.map(x => new ClassB(x)) -
Add the following to the
ClassB
constructor that receivesClassA
as a parameter:case class ClassB () { def this (classA: ClassA) { this (new ClassB (classA.getAttr1, ..., classA.getAttrN)) } }
Solution 5 - Java
Shortcut to convert java list to scala list
import scala.jdk.CollectionConverters._
myjavaList.asScala.toList
Solution 6 - Java
Since scala 2.8.1 use JavaConverters._
to convert scala and Java collections using asScala and asJava methods.
import scala.collection.JavaConverters._
javalist.asScala
scalaSeq.asJava
see the Conversion relationship scala doc site