What role does a Arrays.copyOf plays in making a class as immutable?

Question

An extract/ snippet from Java Concurrency in Practice-

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

    public OneValueCache(BigInteger i, BigInteger[] factors) {
        this.lastNumber = i;
        this.lastFactors = Arrays.copyOf(factors,factors.length);     // 1
    }

    public BigInteger[] getFactors(BigInteger i){
        if(lastNumber == null || lastNumber.equals(i))
            return null;
        else
            return Arrays.copyOf(lastFactors,lastFactors.length);     // 2
    }
}

The author mentions

OneValueCache wouldn't be immutable without the copyOf calls in the constructor and getter. Arrays.copyOf was added as a convenience in Java 6; clone would also work.

1) What role does Arrays.copyOf plays in making the above class as IMMUTABLE? How?

The above immutable class is being utilised like this-

@ThreadSafe
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); // cache being volatile, hence @ThreadSafe
        }

        encodeIntoResponse(resp, factors);
    }

}

Answer 1

If you return this.lastFactors instead of returning a copy, the caller can do (for example)

BigInteger[] lastFactors = cache.getFactors(...);
for (int i = 0; i < lastFactors.length; i++) {
    lastFactors[i] = null;
}

and thus mutate the state of the cache, which is supposed to be immutable.

The explanation is similar for the constructor. If the constructor didn't make a copy, the caller could do

factors = factor(i);
cache = new OneValueCache(i, factors); 
for (int i = 0; i < lastFactors.length; i++) {
    factors[i] = null;
}

and thus once again mutate the state of the cache.

Rule of thumb: an array is always mutable (except the empty array). So, if the state of an immutable class contains an array, then the caller must not be able to have a reference to the array.