Calling Scala's method from Java with Scala's collection type

Question

I have a Java code calling Scala method.

Java side code:

List<String> contexts = Arrays.asList(initialContext);
ContextMessage c = ContextMessage.load(contexts);

Scala side code:

def load(contexts: List[String]) = ...
    contexts foreach context =>

In this case, I have scala.collection.immutable.List<String> cannot be applied ... error message.

I also need to make the type of contexts as general as possible (i.e., Seq) as the load method iterates over the given collection object to process something.

def load(contexts: Seq[String]) = ...

How to solve the two issues?

What's wrong with just using JavaConversions ? This is the standard way to communicate between Java and Scala for collections — Dici
– Dici, Commented Aug 19, 2015 at 23:31
BTW using a Java type in a Scala code is wrong to my opinion, especially List because it has a particular meaning and is associated to specific constructs in Scala — Dici
– Dici, Commented Aug 19, 2015 at 23:33

Dici · Accepted Answer · 2015-08-19 23:41:34Z

2

I would just use JavaConversions and keep my Scala code scalatic.

 // Scala code
object ContextMessage {
    def load(contexts: Seq[String]) = ???
}

// in your Java code
ContextMessage c = ContextMessage.load(JavaConversions.asScalaBuffer(Arrays.asList(initialContext));

answered Aug 19, 2015 at 23:41

Dici

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prosseek · Accepted Answer · 2015-08-20 01:48:54Z

0

In the case Scala calling this method, Implicit conversion between java.util.ArrayList and Seq type can solve this issue easily.

import scala.collection.mutable.ListBuffer

object ContextMessage  extends App  {

    implicit def typeConversion(input: java.util.ArrayList[String]) = {
        val res : ListBuffer[String] = new ListBuffer[String]()
        for (i <- 0 to input.size - 1) {
            // println(input.get(i))
            res += input.get(i)
        }
        res
    }

    def load(contexts: Seq[String]) = {
        contexts foreach { c =>
            println(c)
        }
    }

    val x = new java.util.ArrayList[String]()
    x.add("A")
    x.add("B")
    load(x)
}

ContextMessage.main(args)

The result shows:

A
B

edited Aug 20, 2015 at 1:48

answered Aug 20, 2015 at 1:38

prosseek

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2 Comments

Dici Over a year ago

Every time you call your method you create a new list, it's not great. I have not actually checked what the Scala conversions do, but I guess they just define wrappers around Java objects (I will have a look into that)

Dici Over a year ago

Yeah, that's the case, these are just wrappes, it's much more efficient : github.com/scala/scala/blob/2.11.x/src/library/scala/collection/…

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