Question

I use following complex data structure.

departures = new TreeMap<String,  Map<String,  Set<MyObject>>>();
arrivals=new HashMap<String, Set<MyObject>>();
flights=new HashSet<MyObject>(); 

Then I use loops (I also tried other loops).

for(String dep: nizDep){
    for(String arr: nizArr){
      for(MyObject flight: _flights){
        if(flight.getFrom().equalsIgnoreCase(dep)&&flight.getTo().equalsIgnoreCase(arr)){
                    flights.add(flight);
                 }
                }
                if(!flights.isEmpty()){
            arrivals.put(arr, flights);
                    flights.clear();
                    }
            }
            if(!arrivals.isEmpty()){
            departures.put(dep, arrivals);
            arrivals.clear();
            }
    }
    System.out.println(departures.size()); //result 14
    System.out.println(departures.containsKey("Madrid")); //result true
            arrivals=departures.get("Madrid");
    System.out.println(arrivals.size()); //result 0, arrivals is empty. WHY?

My question is how to use this complex data structure and how to retrieve arrivals from departures?

Answer 1

     System.out.println(arrivals.size()); //result 0, arrivals is empty. WHY?

BECAUSE When you call flights.clear(); after arrivals.put(arr, flights); or arrivals.clear(); after departures.put(dep, arrivals);, this clears your original objects(flights and arrivals). Please bring your initialization statements i.e.

        Map<String, Set<MyObject>> arrivals=new HashMap<String, Set<MyObject>>();
        Set<MyObject>(); flights=new HashSet<MyObject>(); 

within the for loops or replace that statement as below:

                if(!flights.isEmpty()){
                   Set<MyObject> newflights=new HashSet<MyObject>(); 
                    newflights.addAll(flights); //copy elements to new set
                   arrivals.put(arr, newflights);
                    flights.clear();
                }

Same you may do with departures.

Now for retrievals:

      Set<String> arrivalKeys = departures.keySet();
      Interator<String> arrIter = arrivalKeys.iterator();
      while(arrIter.hasNext()){
        String arrKey = arrIter.next();
        Map<String, Set<MyObject>> arrivals = departures.get(arrKey );
        //use your arrivals map object
      }

Same you can do to retrieve flights from arrivals e.g.

for each arrivals retrieved as above:

      Set<String> flightKeys = arrivals.keySet();
      Interator<String> flIter = flightKeys.iterator();
      while(flIter.hasNext()){
        String flKey = flIter.next();
        Set<MyObject> flights = arrivals.get(flKey );
        //use your flights set object
      }