Add Check Support for Java 17 Sealed Classes: ClassFanOutComplexitiy

[mahfouz72]

child of #14969

Check documentation: https://checkstyle.org/checks/metrics/classfanoutcomplexity.html

---

PS D:\CS\test> cat config.xml                                                   
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
        "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
        "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
<module name="Checker">
    <property name="charset" value="UTF-8"/>
    <module name="TreeWalker">
        <module name="ClassFanOutComplexity">
            <property name="max" value="0"/>
        </module>
    </module>
</module>
PS D:\CS\test> cat src/Test.java                                                
public class Test implements I, J {  // violation
}

sealed class C permits D {   // should this be a violation ?
}
final class D extends C {   // violation
}
interface I {
}
interface J {
}
PS D:\CS\test> java  -jar checkstyle-10.17.0-all.jar -c config.xml src/Test.java
Starting audit...
[ERROR] D:\CS\test\src\Test.java:1:1: Class Fan-Out Complexity is 2 (max allowed is 0). [ClassFanOutComplexity]
[ERROR] D:\CS\test\src\Test.java:6:1: Class Fan-Out Complexity is 1 (max allowed is 0). [ClassFanOutComplexity]
Audit done.
Checkstyle ends with 2 errors.
PS D:\CS\test> 

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from the doc:

Checks the number of other types a given class/record/interface/enum/annotation relies on. Also, the square of this has been shown to indicate the amount of maintenance required in functional programs (on a file basis) at least.

I don't think we should add 1 for the classes in the permits list. The super class C permitted certain classes to extend it but it doesn't rely on the permitted subclasses. the opposite is correct

[rnveach]

The super class C permitted certain classes to extend it but it doesn't rely on the permitted subclasses.

I agree.

We definitely need an input for this, and I think documentation has to be added since we declared in documentation how we process things.

[nrmancuso]

We definitely need an input for this, and I think documentation has to be added since we declared in documentation how we process things.

Agreed.