Beust Challenge - Sliding window map

SlidingWindowMap.java

package com.mainaud.slidingwindow;

import java.util.Set;

public class SlidingWindowMap {
    private long periodMs;
    private SlidingWindow[] keyWindow;

    public SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        keyWindow = new SlidingWindow[keys.size()];
        int i = 0;
        for (String key : keys) {
            keyWindow[i++] = new SlidingWindow(key, maxCount);
        }
    }

    public String getNextKey() {
        String key = null;
        // Change i initialization to change key usage distribution
        for (int i = 0; key == null && i < keyWindow.length; ++i) {
            key = keyWindow[i].getKey(periodMs);
        }
        return key;
    }

    /*
     * Keep the last maxCount usage timestamp and test the oldest one. Use an array as a round buffer because it is easy an
     * efficient.
     */
    private static class SlidingWindow {
        private final String key;
        private final long[] timestamp;
        private int next;

        SlidingWindow(String key, int maxCount) {
            this.key = key;
            this.timestamp = new long[maxCount];
            /*
             * works when periodMs < currentTimestamp. if paranoid uncomment next line:
             */
            // java.util.Arrays.fill(timestamp, Long.MIN_VALUE);
        }

        synchronized String getKey(long periodMs) {
            long now = System.currentTimeMillis();
            if (now > timestamp[next] + periodMs) {
                timestamp[next] = now;
                next = (next + 1) % timestamp.length;
                return key;
            } else {
                return null;
            }
        }
    }
}

Many thanks for this, it's really helpful. However, what does 'String key' represent (e.g. line 31)? I can't work it out from your code?