public class PostalCodeDB
{
	// each key is a postal code, its associated value is a set of cities
	private Map codeToCityMap;

	// constructs an empty database
	public PostalCodeDB()
	{
		codeToCityMap = new HashMap();
	}

	// returns the set of cities associated with postalCode if that code is in the database, otherwise returns null
	public Set getCitiesForCode(String code)
	{
		return (Set) codeToCityMap.get(code);
	}

	// adds a new city, postalCode pair to this database
	public void addCityCodePair(String code, String city)
	{
		Set citySet = getCitiesForCode(code);
		if( citySet == null )
		{
			citySet = new HashSet();
			codeToCityMap.put(code, citySet);		
		}
		citySet.add(city);
	}
}