Add Check Support for Java 17 Sealed Classes: RedundantModifer

[mahfouz72]

child of #14969 :

Check documentation : https://checkstyle.org/checks/modifier/redundantmodifier.html#RedundantModifier

---

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="RedundantModifier"/>
    </module>
</module>
PS D:\CS\test> cat src/Test.java                                                
sealed interface I permits A, R {
    public static final String s = null; // 3 violations
    public abstract void m(); // 2 violations
    public static abstract sealed interface J permits A, R {  // 3 violations
       public abstract void m();  // 2 violations
        public static final String s = null;  // 3 violation
    }
}
final class A implements I, I.J {
    @Override
    public void m() {
    }
}
final record R() implements I, I.J {    // violation
    @Override
    public void m() {
    }
}
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:2:5: Redundant 'public' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:2:12: Redundant 'static' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:2:19: Redundant 'final' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:3:5: Redundant 'public' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:3:12: Redundant 'abstract' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:4:5: Redundant 'public' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:4:12: Redundant 'static' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:4:19: Redundant 'abstract' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:5:8: Redundant 'public' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:5:15: Redundant 'abstract' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:6:9: Redundant 'public' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:6:16: Redundant 'static' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:6:23: Redundant 'final' modifier. [RedundantModifier]
[ERROR] D:\CS\test\src\Test.java:14:1: Redundant 'final' modifier. [RedundantModifier]
Audit done.
Checkstyle ends with 14 errors.
PS D:\CS\test> 

---

The check works correctly when the redundant modifiers are on sealed classes. I read the doc and JEP related to sealed classes and I don't find a new case that the modifier is redundant. So, this check doesn't need an update.

[rnveach]

Can you have a final sealed class? Is the final (or even the sealed) redundant if the class is sealed?

permits seems redundant if it is all the classes in the file, but we have discussed another check for this.

[mahfouz72]

we can't have a final sealed class this will not compile as It is not possible for a class to be both sealed (implying subclasses) and final (implying no subclasses)

PS D:\CS\test> cat src/Test.java                                                
public class Test {
}
final sealed class A permits B{
}

final class B extends A {
}
PS D:\CS\test> javac src/Test.java                                              
src\Test.java:3: error: illegal combination of modifiers: final and sealed
final sealed class A permits B{
             ^
src\Test.java:6: error: cannot inherit from final A
final class B extends A {
                      ^
2 errors
PS D:\CS\test> 

[rnveach]

I am good.

[nrmancuso]

Let's add an input file for this.