immutable objects and lazy initialization.

Question

http://www.javapractices.com/topic/TopicAction.do?Id=29

Above is the article which i am looking at. Immutable objects greatly simplify your program, since they:

Answer 1

As explained by others, because the state of the object won't change the hashcode can be calculated only once.

The easy solution is to precalculate it in the constructor and place the result in a final variable (which guarantees thread safety).

If you want to have a lazy calculation (hashcode only calculated if needed) it is a little more tricky if you want to keep the thread safety characteristics of your immutable objects.

The simplest way is to declare a private volatile int hash; and run the calculation if it is 0. You will get laziness except for objects whose hashcode really is 0 (1 in 4 billion if your hash method is well distributed).

Alternatively you could couple it with a volatile boolean but need to be careful about the order in which you update the two variables.

Finally for extra performance, you can use the methodology used by the String class which uses an extra local variable for the calculation, allowing to get rid of the volatile keyword while guaranteeing correctness. This last method is error prone if you don't fully understand why it is done the way it is done...