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

1. Item 20: Prefer class hierarchies to tagged classes

2. Tagged class –

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

3.

4. 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.

5. A tagged class is just imitation of a class hierarchy

Refactor tagged class into a hierarchy

6.

7. Item 21: Use function objects to represent strategies

8. 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

9. 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

10. 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

11. Item 22: Favor static member classes over nonstatic

12. Nested classes

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

13. 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.

14. Anonymous and local classes should be short – ten a fewer lines