Question

I work on a graph where I visualize my emails. I want to be able to get the emails from a certain day.

Is this a bad way to store?

HashMap<DateTime, ArrayList<Email>>

Or is it better to convert the date to a string and then use HashMap<String, ArrayList<Email>>

Note, the dates are added without hours, minutes and seconds, so just like 06/07/2010 for example.

Answer 1

DateTime has properly defined equals and hashcode methods, so using those as the key in a HashMap is perfectly OK. There’s not much to be gained by converting them to strings first.

I would suggest, however, that if you only want to store the year/month/day components, then you may want to use LocalDate instead of DateTime.

Additionally, you could also consider using TreeMap rather than HashMap, so that your map is automatically sorted by date. Might be handy.