Question

I trying to sort this HashMap based on date in keys

My Hash map:

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

Answer 1

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