public class ZipCodeRListImpl 
    implements  ZipCodeRList
{
    String city;
    String zipcode;
    ZipCodeRList next;

    // this constructor creates the terminal of the list.
    // it is assumed this is called at the outset.
    public ZipCodeRListImpl()	
    {
	city = null;
	zipcode = null;
	next = null;
    }

    // this is the standard constructor.
    public ZipCodeRListImpl(String c, String z, ZipCodeRList n)
    {
	city = c;
	zipcode = z;
	next = n;
    }

    // finding the zip code only for that cell.
    // its client can implement recursive search.
    public String find(String c)
    {
	if (c.equals(city))
	    return zipcode;
	else
	    return null;
    }

    // this is essentially cons.
    public ZipCodeRList add(String c, String z)
    {
	return new ZipCodeRListImpl(c, z, this);
    }

    // this is essentially car.
    public ZipCodeRList next()
    {
	return next;
    }
}