I don’t find your logic convincing. People change their contact details, but in software we might choose to retain the old information, maybe marking it as old, for record-keeping purposes. Mutating the contact details loses the old information.

I could make the Person immutable too, and whenever I need to update the Person I create a new Person object and use that from now on.

In my code I often replace setters with ‘withers’, e.g.: netBlock.withNetMask(“255.255.255.0″) creates a new NetBlock with the same info as the original, except with the new NetMask. It’s just an application of the Builder pattern.

@Ricky
Nice factory patternish approach but one downside I guess is that instead of having to maintain one single class, we now have to take care of an interface and an anonymous class.

Yes, wordpress seems to have swallowed the generics type definition . Thanks for pointing that out! Fixed that now.

I would also mention anonymous classes as a way of implementing immutability. If a class has no fields then instances of it are immutable. Of course, anonymous classes generate fields from local variables of the enclosing scope, but they are final.

interface Complex
{
double getReal();
double getImag();
}

public static Complex complex(final double real,final double imag)
{
return new Complex()
{
public double getReal()
{
return real;
}

public double getImag()
{
return imag;
}
};
}

Then you get immutability for no extra effort – you can no longer make the mistake of forgetting ‘final’ on the class definition, as it is implicitly final already.

Not only that, but the perceived effect of implementing an interface anonymously is that nobody thinks “I’d like to subclass that, why can’t I?”.

Cheers,
Mikael