Add CRTP pattern Ref: #2342

crtp/README.md

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+---
+title: Curiously Recurring Template Pattern
+language: en
+category: Structural
+tag:
+- Extensibility
+- Instantiation
+---
+
+## Name / classification
+
+Curiously Recurring Template Pattern
+
+## Also known as
+
+Recursive Type Bound, Recursive Generic
+
+## Intent
+
+Allow derived components to inherit certain functionalities from a base component that are compatible with the derived type.
+
+## Explanation
+
+Real-world example
+
+> 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
+
+> Make certain methods within a type to accept arguments specific to its subtypes.
+
+Wikipedia says
+
+> 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**
+
+Let's define the generic interface Fighter
+
+```java
+public interface Fighter<T> {
+
+  void fight(T t);
+
+}
+```
+
+The MMAFighter class is used to instantiate fighters distinguished by their weight class
+
+```java
+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;
+  }
+
+  @Override
+  public void fight(T opponent) {
+    LOGGER.info("{} is going to fight against {}", this, opponent);
+  }
+
+  @Override
+  public String toString() {
+    return name + " \"" + nickName + "\" " + surname;
+  }
+```
+
+The followings are some subtypes of MmaFighter
+
+```java
+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);
+  }
+
+}
+```
+
+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
+
+```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
+
+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
+
+fighter1.fight(fighter3); // This will raise a compilation error
+```
+
+## Class diagram
+
+![alt text](./etc/crtp.png "CRTP class diagram")
+
+## Applicability
+
+Use the Curiously Recurring Template Pattern when
+
+* 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.
+
+## Tutorials
+
+* [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)
+
+## Known uses
+
+* [java.lang.Enum](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Enum.html)
+
+## Credits
+
+* [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)

crtp/pom.xml

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+<?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>

crtp/src/main/java/crtp/App.java

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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;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * 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 {
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+
+    MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
+    MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
+    fighter1.fight(fighter2);
+
+    MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing");
+    MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu");
+    fighter3.fight(fighter4);
+
+  }
+}

crtp/src/main/java/crtp/Fighter.java

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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;
+
+/**
+ * Fighter interface.
+ *
+ * @param <T> The type of fighter.
+ */
+public interface Fighter<T> {
+
+  void fight(T t);
+
+}

crtp/src/main/java/crtp/MmaBantamweightFighter.java

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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;
+
+/**
+ * MmaBantamweightFighter class.
+ */
+class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> {
+
+  public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) {
+    super(name, surname, nickName, speciality);
+  }
+
+}

crtp/src/main/java/crtp/MmaFighter.java

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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;
+
+import lombok.Data;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * MmaFighter class.
+ *
+ * @param <T> MmaFighter derived class that uses itself as type parameter.
+ */
+@Slf4j
+@Data
+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;
+
+  @Override
+  public void fight(T opponent) {
+    LOGGER.info("{} is going to fight against {}", this, opponent);
+  }
+
+}