Prefer class hierarchies to tagged classes. (Item 20, 21, 22) презентация

Tagged class –
instances come in two or more flavors;
contain a tag field indicating the flavor of the instance

Tagged classes:
Bad readability
Spend to much memory (contain fields belonging to other flavors)
Fields can’t be final (constructors don’t initialize irrelevant fields)
Constructors must set the tag field and initialize the right data fields – compiler can’t control this.
Type doesn’t gives a clue to its flavor.

A tagged class is just imitation of a class hierarchy
Refactor tagged class into a hierarchy

Item 21: Use function objects to represent strategies

Function object is –
An instance of a class that exports exactly one method performing operations on other objects, passed explicitly to the method
Example: Comparator

Concrete strategy
Comparator is concrete strategy for comparison
Typical concrete strategy classes are stateless:
has no fields => all instances of the class are functionally equivalent => should be a singleton

Concrete strategy classes are often declared as anonymous classes
Note that using an anonymous class will create a new instance each time the call is executed
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

Item 22: Favor static member classes over nonstatic

Nested classes
Static member classes
Nonstatic member classes
Anonymous classes
Local classes
Inner classes

If you declare a member class
that does not require access to an enclosing instance –
always put the static modifier in its declaration
Nonstatic static member class has reference to enclosing instance:
it costs time and resources;
it makes enclosing instance not available for garbage collection.