Effective Java - Chapter 6 Enums & Annotations

1. Use enums instead of int constants
   - instance controlled
   - a generalization of singletons
   - 可提供各種物件化的programming
   - enum的method多被為private or package private
   - constant-specific method implementations
   - strategy enum pattern
   - valueOf, toString(), fromString()
2. Use EnumSet, EnumMap
3. Annotation
   - Retention
   - Target
   - Parameter
4. Override
5. Mark interface