Why is this static final variable in a singleton thread-safe?

Question

Reading this site, I\'ve found this:

[The] line private static final Foo INSTANCE = new Foo(); is only executed when the class is actual

Answer 1

The static initialisation block of any class is guaranteed to be single threaded. A simpler singleton is to use an enum

enum Singleton {
    INSTANCE;
}

This is also threads safe and the class lazy-initialised.