How to use scala.collection.immutable.List in a Java code

Question

I need to write a code that compares performance of Java\'s ArrayList with Scala\'s List. I am having a hard time getting the Scala List

Answer 1

It's easier to use Java collections in Scala than the other way around, but since you asked:

import scala.collection.immutable.*;

public class foo {
  public List test() {
    List nil = Nil$.MODULE$; // the empty list
    $colon$colon one = $colon$colon$.MODULE$.apply((Integer) 1, nil); // 1::nil
    $colon$colon two = $colon$colon$.MODULE$.apply((Integer) 2, one); // 2::1::nil
    System.out.println(one);
    System.out.println(two);
    return two;
  }
}

This compiles with javac with scala-library.jar in the classpath:

javac -classpath /opt/local/share/scala-2.9/lib/scala-library.jar foo.java

You can invoke from the Scala REPL:

scala> (new foo).test
List(1)
List(2, 1)
res0: List[Any] = List(2, 1)

To use a Java collection from Scala, you don't have to do anything special:

scala> new java.util.ArrayList[Int]
res1: java.util.ArrayList[Int] = []

scala> res1.add(1)
res2: Boolean = true

scala> res1
res3: java.util.ArrayList[Int] = [1]