singletons - why is the nested class implementation important

http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom is
the recommended solution. My friend Rohan pointed out why this
solution is sometimes necessary.

Say you can't eager init the static field because it must wait for
some service object.

Say you also don't want to incur the static synchronization every
time. Though un-contended lock acquisition is cheap, it's never free.