【设计模式】JAVA Design Patterns——Curiously Recurring Template Pattern（奇异递归模板模式）

🔍目的

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允许派生组件从与派生类型兼容的基本组件继承某些功能。

🔍解释

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真实世界例子

对于正在策划赛事的综合格斗推广活动来说，确保在相同重量级的运动员之间组织比赛至关重要。这样可以防止体型明显不同的拳手之间的不匹配，例如重量级拳手与雏量级拳手的对决。

通俗描述

使类型中的某些方法接受特定于其子类型的参数。

维基百科

奇异递归模板模式（curiously recurring template pattern，CRTP）是C++模板编程时的一种惯用法：其中类X派生自使用X本身作为模板参数的类模板实例化。

程序实例

定义一个通用接口Fighter

public interface Fighter<T> {void fight(T t);}

定义MmaFighter类用于实例化按重量级别区分的拳手

public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> {private final String name;private final String surname;private final String nickName;private final String speciality;public MmaFighter(String name, String surname, String nickName, String speciality) {this.name = name;this.surname = surname;this.nickName = nickName;this.speciality = speciality;}@Overridepublic void fight(T opponent) {LOGGER.info("{} is going to fight against {}", this, opponent);}@Overridepublic String toString() {return name + " \"" + nickName + "\" " + surname;}

 定义MmaFighter 的一些子类型

class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> {public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) {super(name, surname, nickName, speciality);}}public class MmaHeavyweightFighter extends MmaFighter<MmaHeavyweightFighter> {public MmaHeavyweightFighter(String name, String surname, String nickName, String speciality) {super(name, surname, nickName, speciality);}}

效果： 允许拳手与相同重量级的对手交手，如果对手是不同重量级，则会出现错误

MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
fighter1.fight(fighter2); // This is fineMmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing");
MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu");
fighter3.fight(fighter4); // This is fine toofighter1.fight(fighter3); // This will raise a compilation error

🔍类图

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🔍适用场景

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在以下情况下使用CRTP

• 在对象层次结构中链接方法时存在类型冲突
• 您想使用一个参数化的类方法，该方法可以接受类的子类作为参数，从而可以应用于继承自类的对象
• 您希望某些方法仅适用于相同类型的实例，例如实现相互比较。

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