Question

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sort array list of special strings, by date

I have an ArrayList of Strings that read in from a file and the first part of each string is a date ex: 12/01/2012. When I use Collections.sort(); it sorts it fine within the year from oldest to newest, but when it gets to 01/01/2013 it throws that up on the top of the list with the oldest. How can I get it to use the year as part of the sort? I tried using Date, but it did the same. Each string has a lot of info I would like to keep in it and just sort it by the beginning. I see no need to post code since it’s working and all I’m having trouble with is just sorting. I’ve searched on here for awhile and tried a few different sorting options to no avail. So I must be confused or forgetting something. If anyone has any ideas how to sort Strings with dates in the beginning please help! Thanks!

example string: “12/01/2012 34023843 Item Number”

Answer 1

You need a custom comparator. Comparing strings in natural order (ie, using the fact that String implements Comparable<String>) will give the result you see, since 0 is lower than 1, so this is expected.

Here is an example class which can do what you want, but note that it assumes all dates are correctly formatted (the parse() method of DateFormat throws an unchecked IllegalArgumentException if the date is incorrect):

public final class MyComparator
    implements Comparator<String>
{
    // Date parsing
    // Note: SimpleDateFormat is not thread-safe, if possible use Joda Time instead
    private static final DateFormat fmt = new SimpleDateFormat("dd/MM/YYYY");

    private static final Comparator<String> INSTANCE = new MyComparator();

    private MyComparator()
    {
    }

    public static Comparator<String> getInstance()
    {
        return INSTANCE;
    }

    @Override
    public int compare(String o1, String o2)
    {
        // Grab dates
        String s1 = o1.subString(0, 10);
        Date d1 = fmt.parse(s1);
        String s2 = o2.subString(0, 10);
        Date d2 = fmt.parse(s2);

        // Date implements Comparable<Date>, so we can use that...
        int ret = d1.compareTo(d2);

        // If dates are equal, compare the rest of the strings instead.
        return ret != 0 ? ret : o1.subString(10).compareTo(o2.subString(10));
    }
}

You can then use Collections.sort(theArray, MyComparator.getInstance());