Sort objects in ArrayList by date?

Question

Every example I find is about doing this alphabetically, while I need my elements sorted by date.

My ArrayList contains objects on which one of the datamembers is a

Answer 1

This may be an old response but I used some examples from this post to create a comparator that would sort an ArrayList of HashMap by one object in the list, that being the timestamp.

I have these objects:

ArrayList> alList = new ArrayList>();

The map objects are as follows:

Map map = new HashMap<>();
        // of course this is the actual formatted date below in the timestamp
        map.put("timestamp", "MM/dd/yyyy HH:mm:ss"); 
        map.put("item1", "my text goes here");
        map.put("item2", "my text goes here");

That mapping is what I use to load all my objects into the array list, using the alList.add(map) function, within a loop.

Now, I created my own comparator:

import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;

 public class DateSorter implements Comparator {
     public int compare(Object firstObjToCompare, Object secondObjToCompare) {
    String firstDateString = ((HashMap) firstObjToCompare).get("timestamp");
    String secondDateString = ((HashMap) secondObjToCompare).get("timestamp");

    if (secondDateString == null || firstDateString == null) {
        return 0;
    }

    // Convert to Dates
    DateTimeFormatter dtf = DateTimeFormat.forPattern("MM/dd/yyyy HH:mm:ss");
    DateTime firstDate = dtf.parseDateTime(firstDateString);
    DateTime secondDate = dtf.parseDateTime(secondDateString);

    if (firstDate.isAfter(secondDate)) return -1;
    else if (firstDate.isBefore(secondDate)) return 1;
    else return 0;
    }
}

I can now just call the Comparator at any time on the array and it will sort my array, giving me the Latest timestamp in position 0 (top of the list) and the earliest timestamp at the end of the list. New posts get put to the top basically.

Collections.sort(alList, new DateSorter());

This may help someone out, which is why I posted it. Take into consideration the return statements within the compare() function. There are 3 types of results. Returning 0 if they are equal, returning >0 if the first date is before the second date and returning <0 if the first date is after the second date. If you want your list to be reversed, then just switch those two return statements! Simple =]