list - NullPointerException on a listener pattern in scala -

i'm trying create listener design pattern that:

abstract class listener(g: engine) {   g.addlistener(this) } class listener1(g: engine) extends listener(g) class listener2(g: engine) extends listener(g)  class engine {   val listener1 = new listener1(this)   val listener2 = new listener2(this)      var listeners: list[listener] = nil   def addlistener(g: listener) = {     listeners = g::listeners   } } 

but if fails nullpointerexception, because listeners equal null when listener1 , listener2 created.

how overcome problem?

edit: tried following:

  def addlistener(g: listener) = {     if(listeners == null) {       listeners = list(g)     } else {       listeners = g::listeners     }   } 

but problem after class initialized, listeners = nil. need better design pattern achieve goal.

solution: without using lazy val or mutable collections:

var listeners: list[listener] = _ def addlistener(g: listener) = {   if(listeners == null)      listeners = list(g)   else     listeners = g::listeners } 

using _ initialization instead of 'null' or 'nil' makes compiler not overwrite listeners after listener1 , listener2 have been created.

with of lazy value , mutable collection:

class engine {   val listener1 = new listener1(this)   val listener2 = new listener2(this)      lazy val listeners = collection.mutable.listbuffer.empty[listener]   def addlistener(g: listener) {     listeners += g   } } 

if badly need immutable list somewhere down road, turn buffer .tolist method in callplace.