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Prefer class hierarchies to tagged classes. (Item 20, 21, 22), слайд №1

Prefer class hierarchies to tagged classes. (Item 20, 21, 22), слайд №2

Prefer class hierarchies to tagged classes. (Item 20, 21, 22), слайд №3

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Prefer class hierarchies to tagged classes. (Item 20, 21, 22), слайд №5

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Prefer class hierarchies to tagged classes. (Item 20, 21, 22), слайд №14

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Item 20: Prefer class hierarchies to tagged classes

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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

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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

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Item 22: Favor static member classes over nonstatic

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Nested 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.