compacting java code, and style

An un-named library included this joyous piece of sample code (please ignore the gratutious 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?