Scala implementation of mutable stack throwing exception -

here mutable implementation of stack :

class linkedstackgeneric {    var first : nodegeneric = _    def isempty : boolean  = {     first == null   }    def push(itemname : any) = {     val oldfirst = first     first = new nodegeneric(itemname , oldfirst)   }     def pop = {     first = first.next      first.itemname    }  }        class nodegeneric(val itemname : , val next : nodegeneric) {}     println("*********** testing linkedstackgeneric *****************")         val generic = new linkedstackgeneric        generic.push("test")        generic.push("here");        println(generic.isempty)        println(generic.pop);        println(generic.isempty)        println(generic.pop);        println(generic.isempty) 

when run code produces following :

*********** testing linkedstackgeneric ***************** false test false exception in thread "main" java.lang.nullpointerexception     @ linkedstackgeneric.pop(linkedstackgeneric.scala:19)     @ stacksandqueuestest$.main(stacksandqueuestest.scala:37)     @ stacksandqueuestest.main(stacksandqueuestest.scala) 

this line causes error :

println(generic.pop); 

what causing exception, not pushing items onto stack correctly ? since push 2 items :

   generic.push("test")    generic.push("here"); 

shouldn't exception not thrown ?

fix pop method. order of execution causing issue...

 def pop = {     val ret  = first.itemname     first = first.next     ret   }