public final void add_target_door(ExpandDoorItem p_door){
  if (p_door == null)   return;
  target_doors.add(p_door);
}

public final void add_target_door ( ExpandDoorItem p_door ) { if ( p_door == null ) return ; target_doors . add ( p_door ) ; }
adds p_door to the list of target doors of this room .
