Private method used from companion object is marked as unused

Rules and Languages Report False-positive / False-negative...
1

Example:

object Test {
  def main(args: Array[String]): Unit = {
    Test().usedMethod()
  }
}

case class Test() {
  private def usedMethod(): Unit = {

  }
}

Sonarqube 8.4.2.36762 reports: Remove this unused private "usedMethod" method

2

Hello @pieter12321,

  • First, could you specify the language ? I am not completely fluent on all 27 languages supported by SonarQube, and I cannot recognize your code language in the blink of an eye…
  • Second, it seems to me that the usedMethod() is only used by the test code, and therefore unused in the production code, which is the purpose of the “unused private methods” rule.

Olivier

4

Hello,

Sorry about that, I thought the scala tag would be enough :). I guess you are inferring this is test code by the name Test, but this is just the name of the class, you could substitute any other name.

5

Thanks for the clarification. I missed the scala tag and indeed I don’t know Scala enough (I don’t know Scala at all is a better statement) to assess that the Test class was not necessarily a test class.

I could reproduce your “problem” (issue raised on “unused private method”). However I am not sure this is really a false positive. I am reaching my limits of comprehension of the Scala language: It seems from the language that a private method within a class should make the method only accessible from within that class. This does not seem to be the case of a case class like in your code sample. I did not find clear documentation on case class that would explain if and how private elements of the class can be accessed from the outside.
Could you clarify that ?

Thanks.

6

Certainly I can clarify: this is a special case in the scala language where a class (case class Test) can have a so-called companion object (object Test): https://docs.scala-lang.org/overviews/scala-book/companion-objects.html

If you’re familiar with Java, a useful analogy is that the declarations in a companion object are much like static declarations in the corresponding class, something like (java):

class Test {
  static void main(String[] args);
  private void usedMethod();
}
7

Thanks, that clarifies and that sounds like a false positive at first sight then.
But I really need to discuss with our Scala experts before coming back with a final statement on this.
We’ll keep you posted.

9

Thanks for this good catch. This is a false positive.

I’ve created a ticket, to fix it. Here you can find the link https://jira.sonarsource.com/browse/SONARSLANG-500

Have a nice day!