Converting a Java collection into a Scala collection

Question

Related to Stack Overflow question Scala equivalent of new HashSet(Collection) , how do I convert a Java collection (java.util.List say) into a Scala collection List?

I am actually trying to convert a Java API call to Spring's SimpleJdbcTemplate, which returns a java.util.List<T>, into a Scala immutable HashSet. So for example:

val l: java.util.List[String] = javaApi.query( ... )
val s: HashSet[String] = //make a set from l

---

This seems to work. Criticism is welcome!

import scala.collection.immutable.Set
import scala.collection.jcl.Buffer 
val s: scala.collection.Set[String] =
                      Set(Buffer(javaApi.query( ... ) ) : _ *)

Solution 1

Your last suggestion works, but you can also avoid using jcl.Buffer:

Set(javaApi.query(...).toArray: _*)

Note that scala.collection.immutable.Set is made available by default thanks to Predef.scala.

Solution 2

For future reference: With Scala 2.8, it could be done like this:

import scala.collection.JavaConversions._
val list = new java.util.ArrayList[String]()
list.add("test")
val set = list.toSet

set is a scala.collection.immutable.Set[String] after this.

Also see Ben James' answer for a more explicit way (using JavaConverters), which seems to be recommended now.

Solution 3

If you want to be more explicit than the JavaConversions demonstrated in robinst's answer, you can use JavaConverters:

import scala.collection.JavaConverters._
val l = new java.util.ArrayList[java.lang.String]
val s = l.asScala.toSet

Solution 4

JavaConversions (robinst's answer) and JavaConverters (Ben James's answer) have been deprecated with Scala 2.10.

Instead of JavaConversions use:

import scala.collection.convert.wrapAll._

as suggested by aleksandr_hramcov.

Instead of JavaConverters use:

import scala.collection.convert.decorateAll._

For both there is also the possibility to only import the conversions/converters to Java or Scala respectively, e.g.:

import scala.collection.convert.wrapAsScala._

Update: The statement above that JavaConversions and JavaConverters were deprecated seems to be wrong. There were some deprecated properties in Scala 2.10, which resulted in deprecation warnings when importing them. So the alternate imports here seem to be only aliases. Though I still prefer them, as IMHO the names are more appropriate.

Solution 5

You may also want to explore this excellent library: scalaj-collection that has two-way conversion between Java and Scala collections. In your case, to convert a java.util.List to Scala List you can do this:

val list = new java.util.ArrayList[java.lang.String]
list.add("A")
list.add("B")
list.asScala

View more solutions

Comments

[oxbow_lakes]

This isn't great because my java.util.List is coming back out of a Java API as a parametrized list (so my call to the API yields a java.util.List<String>) - I'm trying to turn this into a scala immutable HashSet

[oxbow_lakes]

This suggestion doesn't work where I want to keep the type information

[Michael Neale]

That library is the greatest thing ever. Really really works well.

[Surya Suravarapu]

Revisiting this after couple of years, Scala's JavaConverters is the way to go.

[asiviero]

Looks like now JavaConversions has some implicits making the toSet call not necessary.

[krookedking]

@Rajish I think it's better if the conversion is explicit (see stackoverflow.com/questions/8301947/…)

[ajkl]

this is a great answer and solved my problem. Most of the methods mentioned on the internet just convert the (eg) java.util.List[java.lang.Long] to scala.collection.immutable.List[java.lang.Long]. So this method worked for me to convert it to scala.collection.immutable.List[scala.Long]

[Joost den Boer]

Why do you say the JavaConversions and JavaConverters are deprecated? In the current documentation 2.11.8 both are not marked as being deprecated and they also do not have a reference to 'decorateAll' or 'wrapAll' stating that is the new and preferred usage.

[stempler]

@JoostdenBoer Seems there were some deprecated decorators there in Scala 2.10, though they seem to have been removed again with Scala 2.11.

[Abhishek Sengupta]

you are right, its better to use import scala.collection.JavaConverters._