Convert from scala.collection.Seq<String> to java.util.List<String> in Java code

java scala type-conversion scala-java-interop

24,103

Solution 1

You're on the right track using JavaConversions, but the method you need for this particular conversion is seqAsJavaList:

java.util.List<String> convert(scala.collection.Seq<String> seq) {
    return scala.collection.JavaConversions.seqAsJavaList(seq);
}

Update: JavaConversions is deprecated, but the same function can be found in JavaConverters.

java.util.List<String> convert(scala.collection.Seq<String> seq) {
    return scala.collection.JavaConverters.seqAsJavaList(seq);
}

Solution 2

Since Scala 2.9, you shouldn't use implicits from JavaConversions since they are deprecated and will soon be removed. Instead, to convert Seq into java List use convert package like this (although it doesn't look very nice):

import scala.collection.convert.WrapAsJava$;

public class Test {
    java.util.List<String> convert(scala.collection.Seq<String> seq) {
        return WrapAsJava$.MODULE$.seqAsJavaList(seq);
    }
}

Solution 3

Since 2.12 this is the recommended way:

public static <T> java.util.List<T> convert(scala.collection.Seq<T> seq) {
    return scala.collection.JavaConverters.seqAsJavaList(seq);
}

All other methods a @deprecated("use JavaConverters or consider ToJavaImplicits", since="2.12.0")

24,103

Author by

Cacho Santa

#include <stdio.h> int main(int argc, char *argv[]) { printf("90 Hours A Week And Loving It!"); return 0; }

Updated on April 21, 2020

Comments

Cacho Santa about 4 years

I'm calling a Scala method, from Java. And I need to make the conversion from Seq to List.

I can't modified the signature of the Scala method, so I can't used the asJavaCollection method from scala.collection.JavaConversions._

Any ideas of how can I achieve this?

Using Scala 2.9.3
Rüdiger Klaehn almost 11 years

It might be a good idea for scala to provide a more java-friendly set of frontend methods for these kinds of conversions.
Ashish Ratan about 10 years

I'm not able to import scala.collection.JavaConversions; in eclipse. its showing Error here
grinch about 9 years

Not sure if this is new or not but I had to do seqAsJavaList(seq).asJava to get the actual List<E>
Tom Wang over 6 years

The question asks for the conversion in Java code, not Scala code.
rasen58 over 6 years

Ah my mistake, I knew I found it strange that no one had suggested this solution yet - makes sense now.
Decoded about 5 years

Just added an anonymous edit to this point to use the new JavaConverters class (since 2.8.1). Hopefully will be approved.
Chris Martin about 5 years

@Decoded: Thanks for the improvement :) I didn't accept the edit as-is because I wanted the answer to still make sense in the context of the question, but I added an amendment that uses JavaConverters.
Decoded about 5 years

Thanks! Love to be on the cutting edge :D