FinalClass should exempt private-only constructor classes that are extended by another class in the same compilation unit

[JnRouvignac]

$ cat src/main/java/Main.java

public final class Main {
    private class SuperClass {
        private SuperClass() {
        }
    }

    private final class SubClass extends SuperClass {
    }
}

$ cat checkstyle.xml 

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
        "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
    <module name="TreeWalker">
        <module name="FinalClass"/>
    </module>
</module>

$ cat pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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>
  <groupId>test</groupId>
  <artifactId>test</artifactId>
  <version>1.0-SNAPSHOT</version>
  <name>test checkstyle</name>
  <properties>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
  </properties>
  <build>
    <plugins>
     <plugin>
       <groupId>org.apache.maven.plugins</groupId>
       <artifactId>maven-checkstyle-plugin</artifactId>
       <version>3.1.2</version>
          <dependencies>
            <dependency>
              <groupId>com.puppycrawl.tools</groupId>
              <artifactId>checkstyle</artifactId>
              <version>9.0.1</version>
            </dependency>
          </dependencies>
       <configuration>
         <configLocation>checkstyle.xml</configLocation>
         <encoding>UTF-8</encoding>
         <consoleOutput>true</consoleOutput>
         <failsOnError>true</failsOnError>
         <linkXRef>false</linkXRef>
       </configuration>
       <executions>
         <execution>
           <id>validate</id>
           <phase>validate</phase>
           <goals>
             <goal>check</goal>
           </goals>
         </execution>
       </executions>
     </plugin>
    </plugins>
  </build>
</project>

$ mvn clean install 
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------------< test:test >------------------------------
[INFO] Building test checkstyle 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ test ---
[INFO] Deleting /home/jean-noel/git/checkstyle/target
[INFO] 
[INFO] --- maven-checkstyle-plugin:3.1.2:check (validate) @ test ---
[INFO] Starting audit...
[ERROR] /home/jean-noel/git/checkstyle/src/main/java/Main.java:2:5: Class SuperClass
should be declared as final. [FinalClass]
Audit done.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.809 s
[INFO] Finished at: 2021-10-12T12:08:27+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.1.2:check
(validate) on project test: Failed during checkstyle execution: There is 1 error reported
by Checkstyle 9.0.1 with checkstyle.xml ruleset. -> [Help 1]
...

Similarly to #9357, the FinalClass module is doing it's job correctly in that it's detecting a non-final class with only private constructor(s), but it's not smart enough to realize that in this situation, there is another class that extends it in the same java file.
This is allowed because these classes are nestmates.

Following checkstyle advice would result in compilation failure.

[Vyom-Yadav]

@strkkk @rnveach @pbludov Why isn't this approved yet? I will work on it once approved

[pbludov]

I will work on it once approved

Done. I'm able to reproduce this issue with JDK 8 and JDK17 on my PC.

[romani]

@JnRouvignac , please next time report issue as we request it, without any plugin, just our CLI.

[JnRouvignac]

Sorry I was a bit in a hurry. I will next time.

[rnveach]

Fix is merged

[JnRouvignac]

Thank you very much to all involved!