public class VolatileCachedFactorizer implements Servlet {
private volatile OneValueCache cache =
    new OneValueCache(null, null);

public void service(ServletRequest req, ServletResponse resp) {
    BigInteger i = extractFromRequest(req);
    BigInteger[] factors = cache.getFactors(i);
    if (factors == null) {
        factors = factor(i);
        cache = new OneValueCache(i, factors);
    }
    encodeIntoResponse(resp, factors);
}
}

class OneValueCache {
private final BigInteger lastNumber;
private final BigInteger[] lastFactors;

public OneValueCache(BigInteger i,
                     BigInteger[] factors) {
    lastNumber  = i;
    lastFactors = Arrays.copyOf(factors, factors.length);
}

public BigInteger[] getFactors(BigInteger i) {
    if (lastNumber == null || !lastNumber.equals(i))
        return null;
    else
        return Arrays.copyOf(lastFactors, lastFactors.length);
}
}