View difference between Paste ID: 328R0sq9 and n75TcNJX

 package com.xsm.beustchallenge.slidingwindowmap;
 
 import java.util.Date;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
 
 /**
  * Solving for Beust Challenge: Sliding Window Map
  * http://beust.com/weblog/2012/09/02/coding-challenge-a-sliding-window-map/
  *
  * @author Sony Mathew
  */
 public class SlidingWindowMap {
 
 	private final Set<String> keys;
 	private final int maxCount;
 	private final long maxPeriodMs;
 
 	private List<KeyTrack> keyTracks = new LinkedList<>();
 
 	/**
 	 * author Sony Mathew
 	 */
 	public SlidingWindowMap(Set<String> keys, int maxCount, long maxPeriodMs) {
 		this.keys = keys;
 		this.maxCount = maxCount;
 		this.maxPeriodMs = maxPeriodMs;
 
 		for(String key : this.keys) {
 			keyTracks.add(new KeyTrack(key));
 		}
 	}
 
 	/**
 	 * @return a key that has been used less than `maxCount` times during the
 	 *         past `maxPeriodMs` milliseconds or null if no such key exists.
 	 */
 	public synchronized String getNextKey() {
 		List<KeyTrack> keyTracksUsedUp = new LinkedList<>();
 		try {
 			Iterator<KeyTrack> keyTrackIter = keyTracks.iterator();
 			while(keyTrackIter.hasNext()) {
 				KeyTrack keyTrack = keyTrackIter.next();
 				String key = keyTrack.useKey();
 				if (key != null) {
 					return key;
 				}else{
 					keyTrackIter.remove();
 					keyTracksUsedUp.add(keyTrack);
 				}
 			}
 			return null; //no keys found, all used up.
 		}finally{
 			//put used-up keys back at the end.
 			for(KeyTrack keyTrackUsedUp : keyTracksUsedUp) {
 				keyTracks.add(keyTrackUsedUp);
 			}
 		}
 	}
 
 	/**
 	 * Tracks a key's use.
 	 *
 	 * @author Sony Mathew
 	 */
 	class KeyTrack {
 		final String key;
 		final List<Date> useTimes = new LinkedList<>();
 
 		KeyTrack(String key) {
 			this.key = key;
 		}
 
 		/**
 		 * @return : key if it can be used, else null.
 		 * author Sony Mathew
 		 */
 		String useKey() {
 			Date now = new Date();
 
 			//Remove all old use times.
 			Iterator<Date> useTimeIter = useTimes.iterator();
 			while (useTimeIter.hasNext()) {
 				Date useTime = useTimeIter.next();
 				long usePeriod = now.getTime()-useTime.getTime();
 				if (usePeriod>maxPeriodMs) {
 					useTimeIter.remove(); //no longer countable, past max period.
 				}else{
 					break; //list is in order of oldest to newest, so done.
 				}
 			}
 
 			if (useTimes.size() < maxCount) {
 				useTimes.add(now);
 				return key;
 			}
 
 			return null;
 		}
 	}
 }