Sorting an arraylist of objects by date using SS::MM::HH MM/DD/YY/ format

Question

Hi,

I have an arraylist of objects, each containing a unique date in the following format

YEAR-DAY-MONTH HOUR:MINUTE:SECOND (example: 2010-02-10 23:32:14)

I'm trying to compare each object in the array list (there are a few thousand) with a incrementing timer class I created to check if both times match. It seems like the easiest way to check to see if the object and timer have matching dates would be to first sort the array list from earliest to latest, and then check each object individual, going on the next one in the list once one matches the timer. However, I'm not sure how I would sort an array list based on the previously mentioned time format. So far, my dated objects are have the following properties:

public float min;
public float hour;
public float day;
public float month;
public float year;

How could I sort an array list of these objects so that it was sorted from the earliest to latest date?

Answer 1

Potential simple solution

It seems to me that you're reinventing the wheel here. There are plenty of types representing a date and time already - creating your own one is likely to lead to pain. In this case I'd recommend using LocalDateTime from Joda Time.

Answering the actual question

You mention a "format" as if we're really dealing with strings - but I can't see anything about strings there.

Why are you using float values? These seem like natural integers - although you'll want seconds as well.

You could make your type implement Comparable<T> fairly easily and naturally - then you just need to call Collections.sort and it will do the right thing. Here's a sample implementation of Comparable<Foo>, assuming your type is called Foo:

public int compareTo(Foo other)
{
    if (other == null)
    {
        throw new NullPointerException();
    }
    if (year < other.year)
    {
        return -1;
    }
    if (year > other.year)
    {
        return 1;
    }
    if (month < other.month)
    {
        return -1;
    }
    if (month > other.month)
    {
        return 1;
    }
    // etc
    // If we haven't returned after comparing everything
    // then the values must be equal
    return 0;
}

chburd's answer of implementing Comparer<T> instead is equally valid; if your object is just a date/time, then you've got a natural sort order and Comparable<T> is probably a better choice; if you actually have more properties in the object and you may want to sort by those instead, then Comparer<T> would be better.

Answer 2

1 - Create a Comparator for your object

2 - Then, use Collections.sort on a list of these objects

Answer 3

This might be a too simple answer, but why not just put the values into a TreeMap to start with?

    TreeMap<Date, Date> sortedMap = new TreeMap<Date, Date>();
    List<Date> listOfObjectsToSort = new ArrayList<Date>();
    for (Date theDate : listOfObjectsToSort)
    {
      sortedMap.put(theDate.getTime(), theDate);
    }
    List<Date> sotedList = (List<Date>) sortedMap.values();

The treemap can sort on the long of the time.

I know its more expensive to create the map of the list, when you could have a Comparator on the arraylist.

(now off to get coffee and then read this again)