How to sort HashMap based on Date?

Question

I trying to sort this HashMap based on date in keys

My Hash map:

Map<Date, ArrayList> m = new HashMap<Date, ArrayList>();

Solution

Use a TreeMap instead of HashMap. As Date already implements Comparable, it will be sorted automatically on insertion.

Map<Date, ArrayList> m = new TreeMap<Date, ArrayList>();

Alternatively, if you have an existing HashMap and want to create a TreeMap based on it, pass it to the constructor:

Map<Date, ArrayList> sortedMap = new TreeMap<Date, ArrayList>(m);

See also:

• Java tutorials - Map implementations
• Java tutorials - Object ordering

Comments

[BalusC]

Welcome at Stack Overflow! Just curious, why are you repeating an already given answer? This isn't kind of a "discussion forum" where folks usually confirm the answer by repeating it more or something. Here on a Q&A site you just vote it up or post a better answer. See also stackoverflow.com/faq.

[user1066566]

Yeah,I got it.I will do better.

[Skynet]

If I am implementing Sort on dates, the above method will sort them based on first day of the year. While I want a sort based on the current date, I will have to implement a comparator. Any heading?

[BalusC]

@Pirate: It's sorted on timestamp (as available by Date#getTime()), not on first day of year. Even more, the java.util.Date doesn't have any concept like "first day of year". Your problem is caused elsewhere. Perhaps Date instances used as keys were created the wrong way via java.util.Calendar which indeed has a concept of "first day of year". Just press "Ask Question" button on right top along with an SSCCE in order to be able to get answers on that.

[Skynet]

So you mean it is sorted based on the current date?

[BalusC]

@Pirate: It's sorted on timestamp, not on first day of year as you implied. So it's not e.g. 1 Jan 2012, 1 Jan 2013, 2 Jan 2012, 2 Jan 2013, but it's 1 Jan 2012, 2 Jan 2012, 1 Jan 2013, 2 Jan 2013.

[Skynet]

Means I do not have to implement a comparetor, however as these date values are stored as keys, do they need to be unique? I am about to find this all out, writing a SSCCE.

[Narendra Singh]

How to sort it in descending order?

[BalusC]

@MrNarendra: just use docs.oracle.com/javase/8/docs/api/java/util/…