public long getChecksum() throws IOException {
public synchronized long getChecksum() throws IOException {
    public long getChecksum() throws IOException {        // delay checksum check if already checked recently        if (System.currentTimeMillis() > lastChecksumTime + cacheTime) {            update();            long checksum = lastModified;            for (int i = 0; i < repositories.length; i++) {                checksum += repositories[i].getChecksum();            }            lastChecksum = checksum;            lastChecksumTime = System.currentTimeMillis();        }        return lastChecksum;    }