Qualifing to be an immutible object

Question

Question: Is it possible to design an immutable object having final object references to mutable objects and still be 'viewed' as immutable.
From the book JAVA Concurrency in Practice by Brian Goetz the following statements are made (that have confused me a bit):
"Immutable objects ... can be safely accessed even when synchronization is not used to publish the object reference." (p.51)
"... if [an object's] final fields refer to mutable objects, synchronization is still required to access the state of the objects they refer to."(p.52) To me this states that this type of object is not considered immutable.
However, notice the final array object references to BigInteger[] from an example on page 49:
@Immutable
class OneValueCache {
    private final BigInteger lastNumber;
    private final BigInteger[] lastFactors;
    public OneValueCache( BigInteger i, BigInteger[] factors ) {
        lastNumber = i;
        lastFactors = Array.copyOf( factors, factors.length );
    public BigInteger[] getFactors( BigInteger i ) {
        if ( lastNumber == null || !lastNumber.equals( i ))
            return null;
        else
            return Arrays.copyOf( lastFactors, lastFactors.length );
} Can all of the bigInteger[] lastFactors be considered final since the array object reference is final? Or is the immutability allowed since lastFactors is isolated with Arrays.copyOf() in both the constructor and getFactors() function? Does isolation of mutable objects references using techniques such as object cloning allow the object design to be immutable?
Edited by: rbroersma on May 18, 2010 10:37 PM

Answer 1

ejp wrote:
So if the Object doesn't exist until the constructor returns what is the constructor is working on? Bringing the object into existence.He means, what is the constructor operating on or +manipulating, i.e. what is "this" referring to at that point?  Because you can pass "this" to other methods as if it existed.  And they can operate on it as if it existed...  So if you can do all those things to an object that doesn't exist, what good is your definition of existence?