Simple lock free stack in Java -

i implemented lock free stack follows:

import java.util.concurrent.atomic.atomicreference; /**  * @author shuang.gao  date: 2017/4/6 time: 14:39  */ public class stack<e> {     private atomicreference<node<e>> head;     public stack() {         head = new atomicreference<>();     }         public void push(e e) {         node<e> node = new node<>();         node.item = e;         (;;) {             node<e> headnode = head.get();             node.next = headnode;             if (head.compareandset(headnode, node)) {                 system.out.println("push: " + e);                 break;             }         }     }     public e pop() {         node<e> headnode;         (;;) {             headnode = head.get();             if (headnode == null) {                 system.out.println("pop: null");                 return null;             }             if (head.compareandset(headnode, headnode.next)) {                 break;             }         }         system.out.println("pop: " + headnode.item);         return headnode.item;     }     private class node<e> {         private node<e> next;         private e item;     } }     

lock free , no aba problem. there problem? thanks!