How to sort Date which is in string format in java?

Question

I have an arraylist with string values like

ArrayList<String> datestring=new ArrayList<String>();
datestring.add("01/21/2013 @03:13 PM");
datestring.add("01/21/2013 @04:37 PM");
datestring.add("01/21/2013 @10:41 AM");
datestring.add("01/21/2013 @10:48 AM");
datestring.add("01/22/2013 @06:16 AM");
datestring.add("01/22/2013 @06:19 AM");
datestring.add("01/21/2013 @05:19 PM");
datestring.add("01/21/2013 @05:19 PM");

Can any body help me on sorting the above list? So that the values are sorted according to AM and PM format.
The expected output after sorting should be

for (String s : datestring)
{
    System.out.println(s);
}

.

01/21/2013 @10:41 AM;
01/21/2013 @10:48 AM;
01/21/2013 @03:13 PM;
01/21/2013 @04:37 PM;
01/21/2013 @05:16 PM;
01/21/2013 @05:19 PM;
01/22/2013 @06:16 AM;
01/22/2013 @06:19 AM;

Answer 1

try this

    Collections.sort(datestring, new Comparator<String>() {
        DateFormat f = new SimpleDateFormat("MM/dd/yyyy '@'hh:mm a");
        @Override
        public int compare(String o1, String o2) {
            try {
                return f.parse(o1).compareTo(f.parse(o2));
            } catch (ParseException e) {
                throw new IllegalArgumentException(e);
            }
        }
    });

or with Java 8

    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy '@'hh:mm a");
    Collections.sort(datestring, (s1, s2) -> LocalDateTime.parse(s1, formatter).
            compareTo(LocalDateTime.parse(s2, formatter)));

Answer 2

One can compare the List of date in string format by using the compareTo as given below.

ArrayList<String> datestring=new ArrayList<String>();
datestring.add("01/21/2013 @03:13 PM");
datestring.add("01/21/2013 @04:37 PM");
datestring.add("01/21/2013 @10:41 AM");
datestring.add("01/21/2013 @10:48 AM");
datestring.add("01/22/2013 @06:16 AM");
datestring.add("01/22/2013 @06:19 AM");
datestring.add("01/21/2013 @05:19 PM");
datestring.add("01/21/2013 @05:19 PM");

Collections.sort(datestring, new Comparator<String>() {
                @Override
                public int compare(String object1, String object2) {
                    return object1.compareTo(object2);
                }
            });

By using this you even dont have to parse the String into date

Answer 3

One option is to add the dates to a TreeMap, with the unformatted date as the key, and the formatted date as the value. Using a TreeMap will sort the values automatically.

private ArrayList<String> sortDates(ArrayList<String> dates) throws ParseException {
    SimpleDateFormat f = new SimpleDateFormat("dd MMM yyyy");
    Map <Date, String> dateFormatMap = new TreeMap<>();
    for (String date: dates)
        dateFormatMap.put(f.parse(date), date);
    return new ArrayList<>(dateFormatMap.values());
}