@Override protected void addToFrontier(Node node){
  if (!isExplored(node)) {
    frontier.add(node);
    updateMetrics(frontier.size());
  }
}

@ Override protected void addToFrontier ( Node node ) { if ( ! isExplored ( node ) ) { frontier . add ( node ) ; updateMetrics ( frontier . size ( ) ) ; } }
inserts the node at the tail of the frontier if the corresponding state is not yet explored .
