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

Сентябрь 6, 2022

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

Содержание

2. Tagged class – instances come in two or more flavors; contain a tag field indicating the
4. Tagged classes: Bad readability Spend to much memory (contain fields belonging to other flavors) Fields can’t
5. A tagged class is just imitation of a class hierarchy Refactor tagged class into a hierarchy
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
9. Concrete strategy Comparator is concrete strategy for comparison Typical concrete strategy classes are stateless: has no
10. Concrete strategy classes are often declared as anonymous classes Note that using an anonymous class will
11. Item 22: Favor static member classes over nonstatic
12. Nested classes Static member classes Nonstatic member classes Anonymous classes Local classes Inner classes
13. If you declare a member class that does not require access to an enclosing instance –
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Tagged class –
instances come in two or more flavors;
contain a tag

field indicating the flavor of the instance

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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.

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

class into a hierarchy

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Item 21: Use function objects to represent strategies

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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

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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

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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

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Item 22: Favor static member classes over nonstatic

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

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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.