Why can't I push onto stack? Java 6 generics -

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i have no idea why i'm unable simple push generic type of object onto stack. i've decided need figuring out. have:

private void preordertrav(bstnode<k> node) {             if(node != null){             mystack.push(node); //null pointer exception              while(!mystack.isempty()){                 mystack.pop();                 node = node.getleft();                 mystack.push(node.getright());                 mystack.push(node.getleft());                  }              }                }

this being called constructor:

public bstsortedlistiterator(bstnode<k> root) {     preordertrav(root); }

anyone have ideas? error, way:

exception in thread "main" java.lang.nullpointerexception     @ bstsortedlistiterator.preordertrav(bstsortedlistiterator.java:33)     @ bstsortedlistiterator.preordertrav(bstsortedlistiterator.java:31)     @ bstsortedlistiterator.<init>(bstsortedlistiterator.java:43)     @ bstsortedlist.iterator(bstsortedlist.java:130)     @ webdictionary.main(webdictionary.java:135)

if sure getting nullpointerexception

mystack.push(node);

and have mentioned there check 

if(node != null)

only thing can null mystack. or else incomplete information.

exception in thread "main" java.lang.nullpointerexception     @ bstsortedlistiterator.preordertrav(bstsortedlistiterator.java:33)

is mystack.push(node); line number 33 in bstsortedlistiterator.java


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