public Collection<Country> getMostPopulatedCountries() {
        int maxPop = 0;
        Set<Country> countries = new HashSet<Country>();
        for (Country country : getAllCountries()) {
            if (country.getPopulation() == maxPop) {
                countries.add(country);
            } else {
                if (country.getPopulation() > maxPop) {
                    countries.clear();
                    countries.add(country);
                }
            }
        }

        return countries;
    }