A little improve to the first answer.

Map<String, String> map = new HashMap<>() {
    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        map.forEach((key, value) -> {
                    sb.append(key).append(": ");
                    sb.append(value).append(System.lineSeparator());
                }
        );
        return sb.toString();
    }
};