Why is volatile used in double checked locking

Question

From Head First design patterns book, the singleton pattern with double checked locking has been implemented as below:

public class Singleton {
            


        

Answer 1

If you didn't have it, a second thread could get into the synchronized block after the first set it to null, and your local cache would still think it was null.

The first one is not for correctness (if it were you are correct that it would be self defeating) but rather for optimization.