public static String generateKey(int operatorId,long windowId){
  return String.valueOf(operatorId) + CHECKPOINT_KEY_SEPARATOR + String.valueOf(windowId);
}

public static String generateKey ( int operatorId , long windowId ) { return String . valueOf ( operatorId ) + CHECKPOINT_KEY_SEPARATOR + String . valueOf ( windowId ) ; }
generates key from operator id and window id to store unique operator checkpoints
