Grouping map entries with Java 8 v1

gistfile1.java

public static Map<LocalDate, Integer> summarizePeriodsOld(final List<Bike> bikes, final Predicate<Map.Entry<LocalDate, Integer>> entryFilter) {	
  return bikes.stream()
    .filter(Bike::hasMilages)
    .flatMap(bike -> bike.getPeriods().entrySet().stream())			
    .filter(Optional.ofNullable(entryFilter).orElse(entry -> true))	    
    .collect(
      HashMap::new,
      (map, period) -> {				    
        map.merge(period.getKey(), period.getValue(), (val1, val2) -> val1 + val2);
      },
      (map1, map2) -> {			    			    
        map2.forEach((k, v) -> {
          map1.merge(k, v, (val1, val2) -> val1 + val2);
        });
      }
    );
}