Return a hashmap derived from an arraylist

I am trying to return a HashMap with the bus numbers and their stops.

I have already created a method to return a HashSet by passing one of the stops into it.

I feel like it can all come together, but I keep going around in circles.

public class BusRoutes
{

    public HashMap<Integer,ArrayList<String>> routes;

    public void populateRoutes()
    {
        ArrayList<String> stops = new ArrayList<>();
        stops.add("High Street");
        stops.add("Acacia Avenue");
        stops.add("Brown Street");
        stops.add("Broadway");
        stops.add("Station");
        routes.put(32, stops);

        ArrayList<String> stops1 = new ArrayList<>();
        stops1.add("High Street");
        stops1.add("Hospital");
        stops1.add("Brown Street");
        stops1.add("School");
        stops1.add("Station");
        routes.put(13, stops1);
    }

    public BusRoutes() {
        routes = new HashMap<>();
        reader = new InputReader();
        populateRoutes();        
    }
    public void printRoute(int busNum) {
        System.out.println("Bus number " + busNum + " route:");

        for (int i = 0; i < routes.get(busNum).size(); i++) {
            System.out.println(routes.get(busNum).get(i));
        }        

    }

    public void printAllRoutes() {
        for (int i = 0; i < routes.size(); i++) {
            System.out.println(routes.get(i));
        }       
    }

    public HashSet<Integer> getBusesStoppingHere(String aStop) {    
        HashSet<Integer> busNumbers = new HashSet<>();

        for (Integer key : routes.keySet()) {
            if (routes.get(key).contains(aStop)) {
                busNumbers.add(key);
            }
        }

        return busNumbers;
    }

    public HashMap<String, HashSet<Integer>> getStopsAndBusNumbers() {
        HashMap<String, HashSet<Integer>> map = new HashMap<String, HashSet<Integer>>();
        
        // This is where I am stuck

        return map;
    }
}

> I want to return a map derived from routes, in which each stop included in the routes map is a key and each value is a set of bus numbers that have that stop on their route.

I believe this will work do what you want.

• first allocate a record to hold a Stop and Bus number. The Map.entry() method could also be used but the record doesn't cost anything and gives the arguments meaningful names.
• then stream the existing map's entrySet()
• Then using flatMap(), stream the entrySet's stops. Each stop and the associated bus will then be mapped to the record.
• Now it is just a matter of grouping each stop to a set of buses.
• Collectors.groupingBy will also take another collector, in this case Collectors.mapping to extract the Bus number from the Pair instance, and then put in a set.

record Pair(String getStop, Integer getBus) {}

public  Map<String, Set<Integer>> getStopsAndBusNumbers(Map<Integer, List<String>> map) {
  return map.entrySet()
      .stream()
      .flatMap(e->e.getValue().stream().map(stop->new Pair(stop, e.getKey())))
      .collect(Collectors.groupingBy(Pair::getStop,
           Collectors.mapping(Pair::getBus, Collectors.toSet())));
}

When called with the routes map, here will be the results.

Station=[32, 13]
School=[13]
Brown Street=[32, 13]
High Street=[32, 13]
Acacia Avenue=[32]
Broadway=[32]
Hospital=[13]

For this to work smoothly, you should change your types to their defining interface.
So HashMap<String, ArrayList<Integer>> should be declared as Map<String, List<Integer>>, etc. This is not just for this exercise but is standard practice in Java. You can always pass or assign an object to its defining interface but not the converse (without casting) since different implementations of that interface could exist.

Here is a more imperative solution that might have been the approach prior to Java 8. The results are the same although when maps and sets are involved the order of contents may change. It uses a nested loop to first iterate over the keys (buses) and then the values (stops). The heart of the matter is to check if the stop exists. If not, add it with a new HashSet. Then simply continue out of the conditional and add the bus number to the set.

public Map<String, Set<Integer>> getStopsAndBusNumbers2(
        Map<Integer, List<String>> map) {
    Map<String, Set<Integer>> result = new HashMap<>();
    for (Entry<Integer, List<String>> e : map.entrySet()) {
        for (String stop : e.getValue()) {
            if (!result.containsKey(stop)) {
                result.put(stop, new HashSet<Integer>());
            }
            result.get(stop).add(e.getKey()); // add bus for stop
        }
    }
    return result;
}
 


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