Singleton:

How to enforce creating a single object in your program?
- private constructor
- store a pointer to the single instance in a static variable
  - when initialized?
- static method getInstance that returns the single instance

Worry about thread safety
- getInstance synchronized: too slow
- make static variable private static final, and initialize it
statically (in a static {} block, for example).  Initializer won't get
run until the class loaded, which would be the first time getInstance
is called.  Thus, no need for double-checked locking as implied by the
text.