import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class TimeList {

	private Map<String, Long> map = new HashMap<String, Long>();

	public void update(String id, long timestamp) {
		map.put(id, timestamp);
	}

	public void remove(String id) {
		map.remove(id);
	}

	public Set<String> getOlderThan(long timestamp) {
		Set<String> result = new HashSet<String>();

		for (String id : map.keySet()) {
			if (map.get(id) < timestamp) {
				result.add(id);
			}
		}

		return result;
	}


	public static void main(String[] args) {
		TimeList list = new TimeList();

		list.update("a", 1000);
		list.update("b", 2000);
		list.update("c", 3000);
		list.update("a", 4000);
		list.update("d", 5000);
		list.update("a", 6000);
		list.update("a", 7000);
		list.update("a", 8000);

		System.out.println(list.map);
		System.out.println("===========");
		System.out.println(list.getOlderThan(4001));

	}

}