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AddeusExMachinaAntonio Addeo
AddeusExMachina
and
Antonio Addeo
authored
feat: Add CRTP pattern Ref: #2342 (#2580)
* Add crtp directory with builder for object hierarchy example Ref: #2342 * Add space package Ref: #2342 * Add comments for checkstyle Ref: #2342 * Add fight package as second crtp example Ref: #2342 * Add README.md Ref: #2342 * Add use cases examples in README.md for crtp Ref: #2342 * Remove space package Ref: #2342 * Move files from fight package to crtp Ref: #2342 * Add real world example in README.md Ref: #2342 * Update crtp README.md Ref: #2342 * Delete FighterTest Ref: #2342 * Update App and AppTest in crtp Ref: #2342 * Add category and tag in README.md frontmatter Ref: #2342 * Add @DaTa annotation and remove constructor and toString in MmaFighter Ref: #2342 * Add FightTest Ref: #2342 --------- Co-authored-by: Antonio Addeo <[email protected]>
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Diff for: ‎crtp/README.md

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---
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title: Curiously Recurring Template Pattern
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language: en
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category: Structural
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tag:
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- Extensibility
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- Instantiation
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---
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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
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> of the same weight class is crucial. This prevents mismatches between fighters of significantly different sizes, such
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> as a heavyweight facing off against a bantamweight.
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In plain words
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> Make certain methods within a type to accept arguments specific to its subtypes.
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Wikipedia says
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> The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X
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> derives from a class template instantiation using X itself as a template argument.
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**Programmatic example**
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Let's define the generic interface Fighter
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```java
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public interface Fighter<T> {
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void fight(T t);
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}
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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
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public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> {
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private final String name;
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private final String surname;
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private final String nickName;
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private final String speciality;
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public MmaFighter(String name, String surname, String nickName, String speciality) {
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this.name = name;
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this.surname = surname;
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this.nickName = nickName;
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this.speciality = speciality;
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}
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@Override
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public void fight(T opponent) {
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LOGGER.info("{} is going to fight against {}", this, opponent);
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}
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@Override
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public String toString() {
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return name + " \"" + nickName + "\" " + surname;
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}
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```
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The followings are some subtypes of MmaFighter
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```java
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class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> {
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public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) {
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super(name, surname, nickName, speciality);
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}
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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) {
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super(name, surname, nickName, speciality);
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}
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}
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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
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there is an error
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```java
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MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
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MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
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fighter1.fight(fighter2); // This is fine
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MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing");
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MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu");
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fighter3.fight(fighter4); // This is fine too
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fighter1.fight(fighter3); // This will raise a compilation error
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```
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## Class diagram
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![alt text](./etc/crtp.png "CRTP class diagram")
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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
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* 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
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* 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)
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* 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)
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* 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)

Diff for: ‎crtp/etc/crtp.png

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Diff for: ‎crtp/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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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).
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The MIT License
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Copyright © 2014-2022 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.26.0-SNAPSHOT</version>
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</parent>
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<artifactId>crtp</artifactId>
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<dependencies>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<executions>
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<execution>
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<configuration>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.crtp.App</mainClass>
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</manifest>
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</archive>
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</configuration>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>

Diff for: ‎crtp/src/main/java/crtp/App.java

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/*
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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).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package crtp;
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import lombok.extern.slf4j.Slf4j;
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/**
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* Shows the {@link Fighter} fight method call on some implementations of {@link MmaFighter}. Note
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* that fighters can only fight against opponents of their same weight class.
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*/
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@Slf4j
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public class App {
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/**
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* Program entry point.
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*
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* @param args command line args
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*/
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public static void main(String[] args) {
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MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
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MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo");
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fighter1.fight(fighter2);
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MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing");
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MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu");
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fighter3.fight(fighter4);
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}
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}

Diff for: ‎crtp/src/main/java/crtp/Fighter.java

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/*
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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).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package crtp;
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/**
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* Fighter interface.
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*
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* @param <T> The type of fighter.
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*/
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public interface Fighter<T> {
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void fight(T t);
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}

Diff for: ‎crtp/src/main/java/crtp/MmaBantamweightFighter.java

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/*
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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).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package crtp;
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/**
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* MmaBantamweightFighter class.
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*/
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class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> {
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public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) {
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super(name, surname, nickName, speciality);
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}
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}

Diff for: ‎crtp/src/main/java/crtp/MmaFighter.java

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/*
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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).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package crtp;
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import lombok.Data;
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import lombok.extern.slf4j.Slf4j;
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/**
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* MmaFighter class.
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*
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* @param <T> MmaFighter derived class that uses itself as type parameter.
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*/
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@Slf4j
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@Data
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public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> {
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private final String name;
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private final String surname;
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private final String nickName;
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private final String speciality;
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@Override
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public void fight(T opponent) {
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LOGGER.info("{} is going to fight against {}", this, opponent);
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}
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}

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