Question

I have a Scala function foo(bs : Stream[Bar]) : Bat that I need to call from Java code. How do I create the “bs” Stream (Stream[Bar]) in Java and lazily generate its Bar objects?

Answer 1

Depending on what needs to be in the stream, it might be easiest to create a java.util.Iterator and then convert this to a Stream via a scala.collection.Iterator:

import scala.collection.JavaConverters;
import scala.collection.immutable.Stream;

...

List<String> list = new ArrayList<String>();

\\ Fill the list somehow...

Iterator<String> it = list.iterator();

Stream<String> stream = JavaConverters.asScalaIteratorConverter(it)
                                      .asScala().toStream();

The iterator doesn’t have to come from a collection, of course—we can just as easily create an infinite stream by implementing our own iterator:

Stream<String> stream = JavaConverters.asScalaIteratorConverter(
  new Iterator<String>() {
    int i = 0;
    public boolean hasNext() { return true; }
    public void remove() { throw new UnsupportedOperationException(); }
    public String next() { return Integer.toString(i++); }
  }
).asScala().toStream();

It’s not as pretty as something like Stream.iterate(0)(_ + 1).map(_.toString), but it works.