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| Original file line number | Diff line number | Diff line change |
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| --- | ||
| title: Curiously Recurring Template Pattern | ||
| language: en | ||
| category: Structural | ||
| tag: | ||
| - Extensibility | ||
| - Instantiation | ||
| --- | ||
|
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| ## Name / classification | ||
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| Curiously Recurring Template Pattern | ||
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| ## Also known as | ||
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| Recursive Type Bound, Recursive Generic | ||
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| ## Intent | ||
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| Allow derived components to inherit certain functionalities from a base component that are compatible with the derived type. | ||
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| ## Explanation | ||
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| Real-world example | ||
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| > For a mixed martial arts promotion that is planning an event, ensuring that the fights are organized between athletes | ||
| > of the same weight class is crucial. This prevents mismatches between fighters of significantly different sizes, such | ||
| > as a heavyweight facing off against a bantamweight. | ||
| In plain words | ||
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| > Make certain methods within a type to accept arguments specific to its subtypes. | ||
| Wikipedia says | ||
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| > The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X | ||
| > derives from a class template instantiation using X itself as a template argument. | ||
| **Programmatic example** | ||
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| Let's define the generic interface Fighter | ||
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| ```java | ||
| public interface Fighter<T> { | ||
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| void fight(T t); | ||
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| } | ||
| ``` | ||
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| The MMAFighter class is used to instantiate fighters distinguished by their weight class | ||
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| ```java | ||
| public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> { | ||
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| private final String name; | ||
| private final String surname; | ||
| private final String nickName; | ||
| private final String speciality; | ||
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| public MmaFighter(String name, String surname, String nickName, String speciality) { | ||
| this.name = name; | ||
| this.surname = surname; | ||
| this.nickName = nickName; | ||
| this.speciality = speciality; | ||
| } | ||
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| @Override | ||
| public void fight(T opponent) { | ||
| LOGGER.info("{} is going to fight against {}", this, opponent); | ||
| } | ||
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| @Override | ||
| public String toString() { | ||
| return name + " \"" + nickName + "\" " + surname; | ||
| } | ||
| ``` | ||
|
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| The followings are some subtypes of MmaFighter | ||
|
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| ```java | ||
| class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> { | ||
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| public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) { | ||
| super(name, surname, nickName, speciality); | ||
| } | ||
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| } | ||
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| public class MmaHeavyweightFighter extends MmaFighter<MmaHeavyweightFighter> { | ||
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| public MmaHeavyweightFighter(String name, String surname, String nickName, String speciality) { | ||
| super(name, surname, nickName, speciality); | ||
| } | ||
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| } | ||
| ``` | ||
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| A fighter is allowed to fight an opponent of the same weight classes, if the opponent is of a different weight class | ||
| there is an error | ||
|
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| ```java | ||
| 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 fine | ||
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| MmaHeavyweightFighter 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 too | ||
|
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| fighter1.fight(fighter3); // This will raise a compilation error | ||
| ``` | ||
|
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| ## Class diagram | ||
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|  | ||
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| ## Applicability | ||
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| Use the Curiously Recurring Template Pattern when | ||
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| * You have type conflicts when chaining methods in an object hierarchy | ||
| * You want to use a parameterized class method that can accept subclasses of the class as arguments, allowing it to be applied to objects that inherit from the class | ||
| * You want certain methods to work only with instances of the same type, such as for achieving mutual comparability. | ||
|
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| ## Tutorials | ||
|
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| * [The NuaH Blog](https://nuah.livejournal.com/328187.html) | ||
| * Yogesh Umesh Vaity answer to [What does "Recursive type bound" in Generics mean?](https://stackoverflow.com/questions/7385949/what-does-recursive-type-bound-in-generics-mean) | ||
|
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| ## Known uses | ||
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| * [java.lang.Enum](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Enum.html) | ||
|
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| ## Credits | ||
|
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| * [How do I decrypt "Enum<E extends Enum<E>>"?](http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ106) | ||
| * Chapter 5 Generics, Item 30 in [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb) |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,64 @@ | ||
| <?xml version="1.0" encoding="UTF-8"?> | ||
| <!-- | ||
| This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
| The MIT License | ||
| Copyright © 2014-2022 Ilkka Seppälä | ||
| Permission is hereby granted, free of charge, to any person obtaining a copy | ||
| of this software and associated documentation files (the "Software"), to deal | ||
| in the Software without restriction, including without limitation the rights | ||
| to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
| copies of the Software, and to permit persons to whom the Software is | ||
| furnished to do so, subject to the following conditions: | ||
| The above copyright notice and this permission notice shall be included in | ||
| all copies or substantial portions of the Software. | ||
| THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
| IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
| FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
| AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
| LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
| OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
| THE SOFTWARE. | ||
| --> | ||
| <project xmlns="http://maven.apache.org/POM/4.0.0" | ||
| xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" | ||
| xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> | ||
| <modelVersion>4.0.0</modelVersion> | ||
| <parent> | ||
| <groupId>com.iluwatar</groupId> | ||
| <artifactId>java-design-patterns</artifactId> | ||
| <version>1.26.0-SNAPSHOT</version> | ||
| </parent> | ||
| <artifactId>crtp</artifactId> | ||
| <dependencies> | ||
| <dependency> | ||
| <groupId>org.junit.jupiter</groupId> | ||
| <artifactId>junit-jupiter-engine</artifactId> | ||
| <scope>test</scope> | ||
| </dependency> | ||
| </dependencies> | ||
| <build> | ||
| <plugins> | ||
| <plugin> | ||
| <groupId>org.apache.maven.plugins</groupId> | ||
| <artifactId>maven-assembly-plugin</artifactId> | ||
| <executions> | ||
| <execution> | ||
| <configuration> | ||
| <archive> | ||
| <manifest> | ||
| <mainClass>com.iluwatar.crtp.App</mainClass> | ||
| </manifest> | ||
| </archive> | ||
| </configuration> | ||
| </execution> | ||
| </executions> | ||
| </plugin> | ||
| </plugins> | ||
| </build> | ||
| </project> |
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| /* | ||
| * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
| * | ||
| * The MIT License | ||
| * Copyright © 2014-2022 Ilkka Seppälä | ||
| * | ||
| * Permission is hereby granted, free of charge, to any person obtaining a copy | ||
| * of this software and associated documentation files (the "Software"), to deal | ||
| * in the Software without restriction, including without limitation the rights | ||
| * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
| * copies of the Software, and to permit persons to whom the Software is | ||
| * furnished to do so, subject to the following conditions: | ||
| * | ||
| * The above copyright notice and this permission notice shall be included in | ||
| * all copies or substantial portions of the Software. | ||
| * | ||
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
| * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
| * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
| * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
| * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
| * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
| * THE SOFTWARE. | ||
| */ | ||
| package crtp; | ||
|
|
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| import lombok.extern.slf4j.Slf4j; | ||
|
|
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| /** | ||
| * Shows the {@link Fighter} fight method call on some implementations of {@link MmaFighter}. Note | ||
| * that fighters can only fight against opponents of their same weight class. | ||
| */ | ||
| @Slf4j | ||
| public class App { | ||
|
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| /** | ||
| * Program entry point. | ||
| * | ||
| * @param args command line args | ||
| */ | ||
| public static void main(String[] args) { | ||
|
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| MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai"); | ||
| MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo"); | ||
| fighter1.fight(fighter2); | ||
|
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| MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing"); | ||
| MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu"); | ||
| fighter3.fight(fighter4); | ||
|
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| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
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| /* | ||
| * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
| * | ||
| * The MIT License | ||
| * Copyright © 2014-2022 Ilkka Seppälä | ||
| * | ||
| * Permission is hereby granted, free of charge, to any person obtaining a copy | ||
| * of this software and associated documentation files (the "Software"), to deal | ||
| * in the Software without restriction, including without limitation the rights | ||
| * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
| * copies of the Software, and to permit persons to whom the Software is | ||
| * furnished to do so, subject to the following conditions: | ||
| * | ||
| * The above copyright notice and this permission notice shall be included in | ||
| * all copies or substantial portions of the Software. | ||
| * | ||
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
| * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
| * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
| * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
| * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
| * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
| * THE SOFTWARE. | ||
| */ | ||
| package crtp; | ||
|
|
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| /** | ||
| * Fighter interface. | ||
| * | ||
| * @param <T> The type of fighter. | ||
| */ | ||
| public interface Fighter<T> { | ||
|
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| void fight(T t); | ||
|
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| } |
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