Según Joshua Bloch, que de esto sabe un rato, la mejor forma es mediante un Enum.
public enum Foo {
INSTANCE;
}
Joshua Bloch explicó esta alternativa en la charla de I/O Google 2008 his Effective Java Reloaded talk at Google I/O 2008:
The Right Way to Implement a Serializable Singleton
public enum Elvis {
INSTANCE;
private final String[] favoriteSongs =
{ "Hound Dog", "Heartbreak Hotel" };
public void printFavorites() {
System.out.println(Arrays.toString(favoriteSongs));
}
}
Edit: En su libro Effective Java explica lo siguiente:
"This approach is functionally equivalent to the public field approach, except that it is more concise, provides the serialization machinery for free, and provides an ironclad guarantee against multiple instantiation, even in the face of sophisticated serialization or reflection attacks. While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton."