使用缓存时不可变 class

Question

While going through JCIP , I came across the below code snippet which was considered as Immutable . However the explanation provided makes me confused.

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.

Aren't the state vars of OneValueCache themselves Immutable with final and BigInteger being itself immutable ? Why do we need to have Arrays.copyOf if the state of OneValueCache is already Immutable ?

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);
}
}

Answer 1

The final applies to lastFactors , but not to the elements of lastFactors . If the methods returned lastFactors directly without copying it, the caller could modify the elements of lastFactors without any problems, changing the internal state of OneValueCache . With the copy operations, the returned arrays are copies of the final variable, and any modifications will modify the copy of the array, not the immutable instance.