<p>What is an efficient way to implement a singleton design pattern in Java?</p>

Make sure that you really need it. Do a google search for "singleton anti-pattern" to see some arguments against it.
There's nothing inherently wrong with it I suppose, but it's just a mechanism for exposing some global resource/data so make sure that this is the best way. In particular, I've found
dependency injection
(DI) more useful particularly if you are also using unit tests, because DI allows you to use mocked resources for testing purposes.