Design Underground System

class UndergroundSystem {
    
    private Map<String, Map<Integer, Integer>> checkIn;
    private Map<String, Map<Integer, Integer>> checkOut;
    
    public UndergroundSystem() {
        checkIn = new HashMap<>();
        checkOut = new HashMap<>();
    }
    
    public void checkIn(int id, String stationName, int t) {
        if( !checkIn.containsKey( stationName)) {
            checkIn.put(stationName, new HashMap<Integer, Integer>());
        }
        Map<Integer, Integer> map = checkIn.get(stationName);
        map.put(id, t);
        
    }
    
    public void checkOut(int id, String stationName, int t) {
        if( !checkOut.containsKey( stationName)) {
            checkOut.put(stationName, new HashMap<Integer, Integer>());
        }
        Map<Integer, Integer> map = checkOut.get(stationName);
        map.put(id, t);
    }
    
    public double getAverageTime(String startStation, String endStation) {
        double result =0.0;
        Map<Integer, Integer> map1 = checkIn.get(startStation);
        Map<Integer, Integer> map2 = checkOut.get(endStation);
        int count =0;
        for(Map.Entry<Integer, Integer> entry : map1.entrySet()) {
            int startTime = entry.getValue();
            int id = entry.getKey();
            if( map2.containsKey(id)) {
                int endTime = map2.get(id);
                result += endTime - startTime;
                count++;
            }
        }
        return result/count;
    }
}

/**
 * Your UndergroundSystem object will be instantiated and called as such:
 * UndergroundSystem obj = new UndergroundSystem();
 * obj.checkIn(id,stationName,t);
 * obj.checkOut(id,stationName,t);
 * double param_3 = obj.getAverageTime(startStation,endStation);
 */