Question

I have a list of objects called Activity:

class Activity {
   public Date activityDate;
   public double amount;
}

I want to iterate through List, group them by date and return a new list . Here’s what I currently do:

private List<Activity> groupToList(List<Activity> activityList) {
    SimpleDateFormatter sdf = new SimpleDateFormatter("YYYY-MM-DD");
    Map<String,Activity> groupMap = new HashMap<String,Activity>();
    for (Activity a in activityList) {
        String key = sdf.format(a.getActivityDate());
        Activity group = groupMap.get(key);
        if (group == null) {
          group = new Activity();
          groupMap.add(key, group);
        }
        group.setAmount(group.getAmount() + a.getAmount());
    }
    return new ArrayList<Activity>(groupMap.values()); 
}

Is it a WTF to use the DateFormatter in this way?

• I’m using the DateFormatter because each activityDate could have time information.

Answer 1

I would just use the date object itself as the key. If it it bothers you because the date object is mutable, then use its toString() value. No reason to go making formats.

If the issue is that you want to normalize the date by removing the time component, it would be much better to do that withing the Activity object and remove the time component. If the issue is still further that there are potential time zone issues, I would use JodaTime, but there is no object in the JDK currently that represents a pure date without time, so going with a string isn’t outrageous, but it should be hidden behind a method in the Activity object and the fact that it is a date formatted string without a time component should be an implementation detail.