public final synchronized void cancel() {
    cancelled = true;

    synchronized (clones) {
      Iterator<Operation> i = clones.iterator();
      while(i.hasNext()) {
        i.next().cancel();
      }
    }

    wasCancelled();
    callback.receivedStatus(CANCELLED);
    callback.complete();
  }
Set the callback for this instance.
src/main/java/net/spy/memcached/protocol/BaseOperationImpl.java