$ javac CheckstylePrivateMethodTest.java
$ 

$ cat config.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="MissingJavadocMethod">
            <property name="scope" value="public"/>
            <property name="minLineCount" value="2"/>
            <property name="tokens" value="METHOD_DEF"/>
        </module>
    </module>
</module>

$ cat CheckstylePrivateMethodTest.java
public interface CheckstylePrivateMethodTest {
  private static void whyIsMissingJavadocMethodBeingFlagged() {
    System.out.println();
    System.out.println();
    System.out.println();
    System.out.println();
  }
}

$ java -jar checkstyle-8.32-all.jar -c config.xml CheckstylePrivateMethodTest.java
Starting audit...
[ERROR] CheckstylePrivateMethodTest.java:2:3: Missing a Javadoc comment. [MissingJavadocMethod]
Audit done.
Checkstyle ends with 1 errors.