SlidingWindowMap code challenge

import java.util.Set;

// Quick experiment of the SlidingWindowMap code challenge
// Author: Thierry Kormann - thierry.kormann@me.com
public class SlidingWindowMap {

    private String[] _keys;
    private int _maxCount;
    private long _periodMs;

    private long[] _times;
    private int _count;

    public SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        _keys = new String[keys.size()];
        keys.toArray(_keys);
        _maxCount = maxCount;
        _periodMs = periodMs;
        _times = new long[keys.size() * maxCount];
    }

    /**
     * @return a key that has been used less than `maxCount` times during the
     * past `periodMs` milliseconds or null if no such key exists.
     */
    public String getNextKey() {
        if (_count == _times.length) {
            _count = 0;
        }
        long now = System.currentTimeMillis();
        if (now >= _times[_count] + _periodMs) {
            _times[_count] = now;
            return _keys[_count++ % _keys.length];
        }
        return null;
    }
}