closures & moving averages (state)

[Adrian Mocanu]

I'm passing a moving average function during the map phase like this:
  val average= new Sma(window=3)
stream.map(x=> average.addNumber(x))

where
  class Sma extends Serializable { .. }

I also tried to put creation of object average in an object like I saw in 
another post:
 object Average {
    val smaFn = new VSRTXYSimpleMovingAverage[(String, Long)](3)
 }
Every time  average.addNumber is called it is a new instance.
How can I preserve state of average object?

Thanks
-Adrian