class TokenHandler {

    public void addHeader(Map<String, List<String>> headers, String token) {
        String tokenValue = headerCreator.apply(token);

        List<String> values = headers.get(tokenHeader);

        if (null == values) {
            values = singletonList(tokenValue);
        } else {
            values = new ArrayList<>(values);
            values.add(tokenValue);
        }

        headers.put(tokenHeader, values);
    }

    private  TokenHandler(Builder builder);

}