Item 20: Prefer class hierarchies to tagged classes

tag field indicating the flavor of the instance

other flavors)
Fields can’t be final (constructors don’t initialize irrelevant fields)
Constructors

tagged class into a hierarchy

exactly one method performing operations on other objects, passed explicitly

are stateless: has no fields => all instances of the class

using an anonymous class will create a new instance each

If concrete strategy used once – anonymous class
If concrete strategy is designed for repeated use:
Implementation – private static member class
Export – final public static field
Type – strategy interface

access to an enclosing instance – always put the static

Nonstatic static member class has reference to enclosing instance:
it costs time and resources;
it makes enclosing instance not available for garbage collection.

fewer lines