How to convert / cast long to String?

See the reference documentation for the String class: String s = String.valueOf(date);

If your Long might be null and you don’t want to get a 4-letter "null" string, you might use Objects.toString, like: String s = Objects.toString(date, null);

EDIT:

You reverse it using Long l = Long.valueOf(s); but in this direction you need to catch NumberFormatException