Hashing

public class Customer {

    private String firstName;
    private String lastName;
    private Date birthday;
    private String street;
    private String city;
    private String zipcode;

    protected boolean equalFields(Customer rhs) {
        return ObjectUtils.eqFields(firstName, rhs.firstName) &&
               ObjectUtils.eqFields(lastName, rhs.lastName) &&
               ObjectUtils.eqFields(birthday, rhs.birthday) &&
               ObjectUtils.eqFields(city, rhs.city) &&
               ObjectUtils.eqFields(zipcode, rhs.zipcode);
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        return (obj instanceof Customer &&
                ((Customer)obj).equalFields(this));
    }

    @Override
    public int hashCode() {
        int result = 21 + ObjectUtils.hash(birthday);
        result = result * 31 + ObjectUtils.hash(city);
        result = result * 31 + ObjectUtils.hash(firstName);
        result = result * 31 + ObjectUtils.hash(lastName);
        result = result * 31 + ObjectUtils.hash(street);
        return result * 31 + ObjectUtils.hash(zipcode);
    }
}

// this class would be public in a package say utils
class ObjectUtils {
    public static boolean eqFields (Object a, Object b) {
        if (a != null)
            return a.equals(b);
        else
            return (b == null);
    }

    public static int hash(Object obj) {
        return ((obj == null) ? 0 : obj.hashCode());
    }
}