refactor: rename and delete some patterns (#2970)

2026-10-13 12:14:51 +00:00 · 2024-05-23 17:57:46 +03:00
1 parent 399970f8d1
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+---
+title: Curiously Recurring Template Pattern
+language: en
+category: Structural
+tag:
+    - Extensibility
+    - Idiom
+    - Instantiation
+---
+
+## Also known as
+
+* CRTP
+* Mixin Inheritance
+* Recursive Type Bound
+* Recursive Generic
+* Static Polymorphism
+
+## Intent
+
+Curiously Recurring Template Pattern (CRTP) is used to achieve a form of static polymorphism by having a class template derive from a template instantiation of its own class, allowing method overriding and polymorphic behavior at compile time rather than at runtime.
+
+## 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, an error is raised.
+
+```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
+
+* When you need to extend the functionality of a class through inheritance but prefer compile-time polymorphism to runtime polymorphism for efficiency reasons.
+* When you want to avoid the overhead of virtual functions but still achieve polymorphic behavior.
+* In template metaprogramming to provide implementations of functions or policies that can be selected at compile time.
+* 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's 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
+
+* Implementing compile-time polymorphic interfaces in template libraries.
+* Enhancing code reuse in libraries where performance is critical, like in mathematical computations, embedded systems, and real-time processing applications.
+* [java.lang.Enum](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Enum.html)
+
+## Consequences
+
+Benefits:
+
+* Elimination of virtual function call overhead, enhancing performance.
+* Safe reuse of the base class code without the risks associated with multiple inheritances.
+* Greater flexibility and extensibility in compile-time polymorphism scenarios.
+
+Trade-offs:
+
+* Increased complexity in understanding and debugging due to the interplay of templates and inheritance.
+* Can lead to code bloat because each instantiation of a template results in a new class.
+* Less flexibility compared to runtime polymorphism as the behavior must be determined entirely at compile time.
+
+## Related Patterns
+
+* [Factory Method](https://java-design-patterns.com/patterns/factory-method/): Can be used in conjunction with CRTP to instantiate derived classes without knowing their specific types.
+* [Strategy](https://java-design-patterns.com/patterns/strategy/): CRTP can implement compile-time strategy selection.
+* [Template Method](https://java-design-patterns.com/patterns/template-method/): Similar in structure but differs in that CRTP achieves behavior variation through compile-time polymorphism.
+
+## Credits
+
+* [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)
+* [How do I decrypt "Enum<E extends Enum\<E>>"?](http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ106)
@@ -0,0 +1,2 @@
+@startuml
+@enduml
@@ -0,0 +1,2 @@
+@startuml
+@enduml
@@ -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>curiously-recurring-template-pattern</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>
@@ -0,0 +1,52 @@
+/*
+ * 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);
+
+  }
+}
@@ -0,0 +1,36 @@
+/*
+ * 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);
+
+}
@@ -0,0 +1,36 @@
+/*
+ * 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);
+  }
+
+}
@@ -0,0 +1,49 @@
+/*
+ * 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);
+  }
+
+}
@@ -0,0 +1,36 @@
+/*
+ * 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;
+
+/**
+ * MmaHeavyweightFighter.
+ */
+public class MmaHeavyweightFighter extends MmaFighter<MmaHeavyweightFighter> {
+
+  public MmaHeavyweightFighter(String name, String surname, String nickName, String speciality) {
+    super(name, surname, nickName, speciality);
+  }
+
+}
@@ -0,0 +1,36 @@
+/*
+ * 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;
+
+/**
+ * MmaLightweightFighter class.
+ */
+class MmaLightweightFighter extends MmaFighter<MmaLightweightFighter> {
+
+  public MmaLightweightFighter(String name, String surname, String nickName, String speciality) {
+    super(name, surname, nickName, speciality);
+  }
+
+}
@@ -0,0 +1,40 @@
+/*
+ * 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 static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Application test
+ */
+class AppTest {
+
+  @Test
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}
@@ -0,0 +1,74 @@
+/*
+ * 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;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@Slf4j
+public class FightTest {
+
+  /**
+   * A fighter has signed a contract with a promotion, and he will face some other fighters. A list of opponents is ready
+   * but for some reason not all of them belong to the same weight class. Let's ensure that the fighter will only face
+   * opponents in the same weight class.
+   */
+  @Test
+  void testFighterCanFightOnlyAgainstSameWeightOpponents() {
+    MmaBantamweightFighter fighter = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai");
+    List<MmaFighter<?>> opponents = getOpponents();
+    List<MmaFighter<?>> challenged = new ArrayList<>();
+
+    opponents.forEach(challenger -> {
+      try {
+        ((MmaBantamweightFighter) challenger).fight(fighter);
+        challenged.add(challenger);
+      } catch (ClassCastException e) {
+          LOGGER.error(e.getMessage());
+      }
+    });
+
+    assertFalse(challenged.isEmpty());
+    assertTrue(challenged.stream().allMatch(c -> c instanceof MmaBantamweightFighter));
+  }
+
+  private static List<MmaFighter<?>> getOpponents() {
+    return List.of(
+          new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo"),
+          new MmaLightweightFighter("Evan", "Evans", "Clean Coder", "Sambo"),
+          new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing"),
+          new MmaBantamweightFighter("Ray", "Raymond", "Scrum Master", "Karate"),
+          new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu")
+    );
+  }
+
+
+}