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Scala Java Interoperability


Converting Scala Collections to Java Collections and vice versa

When you need to pass a collection into a Java method:

import scala.collection.JavaConverters._
val scalaList = List(1, 2, 3)

If the Java code returns a Java collection, you can turn it into a Scala collection in a similar manner:

import scala.collection.JavaConverters._
val javaCollection = JavaLibrary.getList
val scalaCollection = javaCollection.asScala

Note that these are decorators, so they merely wrap the underlying collections in a Scala or Java collection interface. Therefore, the calls .asJava and .asScala do not copy the collections.


Arrays are regular JVM arrays with a twist that they are treated as invariant and have special constructors and implicit conversions. Construct them without the new keyword.

 val a = Array("element")

Now a has type Array[String].

 val acs: Array[CharSequence] = a
 //Error: type mismatch; found : Array[String] required: Array[CharSequence]

Although String is convertible to CharSequence, Array[String] is not convertible to Array[CharSequence].

You can use an Array like other collections, thanks to an implicit conversion to TraversableLike ArrayOps:

 val b: Array[Int] =

Most of the Scala collections (TraversableOnce) have a toArray method taking an implicit ClassTag to construct the result array:
 //> res1: Array[Int] = Array(0)

This makes it easy to use any TraversableOnce in your Scala code and then pass it to Java code which expects an array.


In this page (written and validated by ) you learned about Scala Java Interoperability . What's Next? If you are interested in completing Scala tutorial, your next topic will be learning about: Scala JSON.

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