Date ranges in collections

[Will Myers]

I have a use case where I want to store date ranges in a collection (an object with a start and end date) and find the relevant one that covers a passed in date. I can obviously do this by having a method on the DateRange object to determine if the passed in date falls in this range and iterate through the collection calling this method on each object but this doesn't seem very efficient. Is there a better way to do this? The collection could potentially be huge....

[Christian Dillinger]

I'd create a class that holds fromDate and toDate (let's call it DateHolder) which implements Comparable. In comparable I'd only compare fromDate or toDate. Then you put your objects into a TreeSet.
For each Date you have to check I'd create a DateHolder instance and fill only that field you used in your impl of Comparable.
Now query your TreeSet either by headSet or tailSet (depending on your imp of comparable).

Explanation: If a day is a certain period it has to be before the end of that period. So you only have to check items where toDate is greater than your current day. headSet or tailSet gives you those periods as a SortedSet. Iterate over it and check if your date is after the beginning of the period.

[Christian Dillinger]

I wrote a simple test...

The DateHolder:

package de.test;

import java.text.SimpleDateFormat;
import java.util.Date;

public class DateHolder implements Comparable<DateHolder> {

	private static SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy");

	private Date begin;
	private Date end;

	public DateHolder() {

	}

	public DateHolder(Date begin, Date end) {
		this.begin = begin;
		this.end = end;
	}

	public void setBegin(Date begin) {
		this.begin = begin;
	}

	public void setEnd(Date end) {
		this.end = end;
	}

	public boolean contains(Date d) {
		if (d.after(begin) && d.before(end)) {
			return true;
		}
		return false;
	}

	@Override
	public int compareTo(DateHolder o) {
		if (end.before(o.end)) {
			return -1;
		}
		return 1;
	}

	@Override
	public String toString() {
		return "DateHolder [" + sdf.format(begin) + " - " + sdf.format(end)
				+ "]";
	}

}

A generator for some periods

package de.test;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

public class DateHolderGenerator {

	public static List<DateHolder> getList() {
		List<DateHolder> rc = new ArrayList<DateHolder>();

		Calendar begin = Calendar.getInstance();
		Calendar end = Calendar.getInstance();

		for (int month = 1; month < 12; month++) {
			for (int day = 19; day < 25; day++) {
				begin.set(2010, month - 1, day - 8);
				end.set(2010, month, day);
				Date b = begin.getTime();
				Date e = end.getTime();
				rc.add(new DateHolder(b, e));
			}
		}

		return rc;
	}
}

And finally the main-class which checks some dates:

package de.test;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Random;
import java.util.SortedSet;
import java.util.TreeSet;

public class DateHolderTest {

	public static void main(String[] args) {
		SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy");

		TreeSet<DateHolder> periods = new TreeSet<DateHolder>();
		List<DateHolder> list = DateHolderGenerator.getList();
		for (DateHolder dh : list) {
			periods.add(dh);
		}

		Calendar cal = Calendar.getInstance();
		cal.set(2010, 0, 1);

		Random rnd = new Random(System.currentTimeMillis());

		for (int run = 0; run < 10; run++) {
			int month = rnd.nextInt(12);
			int day = rnd.nextInt(29);
			cal.set(2010, month, day);
			Date e = cal.getTime();
			DateHolder dh = new DateHolder();
			dh.setEnd(e);			
			SortedSet<DateHolder> candidates = periods.tailSet(dh);
			System.out.println("Check " + e + " Number of candidates: " + candidates.size());
			for (DateHolder period : candidates) {
				System.out.println("  " + period + " contains " + sdf.format(e) + ": "
						+ period.contains(e));
			}
		}

	}
}