@Override
    public void setDomain(final String domain) {
        if (domain != null) {
            cookieDomain = domain.toLowerCase(Locale.ROOT);
        } else {
            cookieDomain = null;
        }
    }
Sets the domain attribute.

@param domain The value of the domain attribute

@see #getDomain
httpclient5/src/main/java/org/apache/hc/client5/http/impl/cookie/BasicClientCookie.java