How and why This code is Thread Safe..?

Question

This is the my code..

@immutable // This is not a standard annotation .Only for Showing that behavior of Class 
class OneValueCached{
    private final BigInteger lastNumber;
    private final BigInteger[] lastFactors;
    public OneValueCached(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);
    }
}

@threadSafe // This is not a standard annotation .Only for Showing that behavior of Class 
public class VolatileCachedFactorizer implements Servlet{
    private volatile OneValueCached cache=new OneValueCached(null, null);

    public void service(ServletRequest req, ServletResponce resp){
        BigInteger i= extractFromRequest(req);
        BigInteger[] factors=cache.getFactors(i);
        if(factors==null){   // ---> line 1
            factors=factor(i);  // --> line 2
            cache=new OneValueCached(i, factors);
        }

        encodeIntoResponse(resp,factors);
    }
}

Why class VolatileCachedFactorizer is thread according to Book But My point is..
1. @ Line 1 if 2 thread coming at same time at that point 1st thread check condition and found factor=null and 2nd thread also check same condition at the same time after 1st thread suspend at line 2nd and found factor=null
And both will be create new OneValueCached Object Then how this code is thread safe.. According to book this is thread safe..

Thank's

Answer 1

It's thread safe because there is never inconsistency between lastNumber and lastFactors which could result in incorrect factoring. It makes no guarantees that the minimum amount of factoring will take place: the OneValueCached can be created more than once, but that's still thread-safe.

Answer 2

The exact notion of "thread-safety" has not been defined here. What do you expect to happen/not happen for this code to be thread-safe?

1. If you are expecting the state of all objects involved (as far as I can see in this code) to be consistent, then it's thread-safe (as @artbristol explains).
2. If you are expecting a single creation of a Cache object when service is called concurrently, then it's not thread-safe.

Again, without the definition of thread-safe behavior for this situation we cannot really tell for sure.