Sort a hashmap by date

Question

This is the my json:

[
{"name": "John Smith","date":"2017-02-02T23:07:09Z","id":"1223234"}
{"name": "John Doe","date":"2015-07-03T21:05:10Z","id":"3242342"},
{"name": "Jane Fan","date":"2016-12-22T13:27:19Z","id":"2123444"}
]

I have already converted the above to hashmap. Now I want to sort the items on the date value.

Are there inbuilt functions that can help me achieve this? Like e.g. sorted in Python

A HashMap, by definition, is not sorted. Put your entries into a list, and sort the list. — JB Nizet, Feb 25 '17 at 17:23
I am sorry if I sound odd but should not a hashmap resemble a dictionary. What am I missing? — Echchama Nayak, Feb 25 '17 at 17:25
@EchchamaNayak: That the JSON is giving you an ordered sequence of items, not a map. This is naturally a `List`, not a `Map`. — Jon Skeet, Feb 25 '17 at 17:25

score 5 · Answer 1 · answered Feb 25 '17 at 17:25

I have already converted the above to hashmap.

I would suggest you don't do that. I suggest you convert it to a List<Person> where each Person has a name, date and ID. (If not Person then some other class.) It makes more sense as a list than a map, because that's really what the JSON shows: a list of objects, each of which has a name, date and ID. I'd personally make the date a java.time.Instant or something similar, but that's a separate matter.

At that point, all you need to do is either implement a Comparator<Person> that compares objects by date, or make Person implement Comparable<Person> in the same way. Using a separate Comparator<Person> feels more appropriate in this case, as it seems equally reasonable to sort a List<Person> by name or ID. Then you just need to call Collections.sort(people, comparator) and you're done. There are plenty of examples on Stack Overflow to help you sort by a particular property.

The key part is to realize that what you've got isn't naturally a map - it's a list.

Sort a hashmap by date

1 Answers1