Compacting Java Code, and Style

An unnamed library included this joyous piece of sample code (please ignore the gratuitous style violations — they were clearly C++ coders):

public String[] getPrefixes(String prefix) {
  Vector<string> v = new Vector<string>();
  for(String s : map.keySet())
  {
    if(s.startsWith(prefix) && !s.equals(prefix))
    {
        v.add(s);
    }
  }

  if(v.size() > 0)
  {
    return (String[])v.toArray(new String[0]);
  }
  else
  {
    return null;
  }
}

I thought, “This is a place where Scala can shine. Let’s try to remove the 99% of this code that’s just boilerplate.” Here’s what I came up with:

def getPrefixes(prefix: String): Array[String] = {
  val keyList = (for (val key <- map.keys if key.startsWith(prefix) && (key != prefix)) yield key).toList
  if (keyList.size > 0) keyList.toArray else null
}

But wait. That’s really terse, but doesn’t look awesome. The first line is too long to completely absorb when skimming, and the last line is questionable style — I wanted a Ruby-style keyList.toArray unless keyList.empty? but Scala uses inline if/else for the a ? b : c operator so it looks weird. Second try, let’s use matching instead:

def getPrefixes(prefix: String): Array[String] = {
  (for (val key <- map.keys if key.startsWith(prefix) && (key != prefix)) yield key).toList match {
    case Nil => null
    case list => list.toArray
  }
}

That works. The (…).toList construct still bothers me a bit though. Is there some better way to do that?