Mark case objects as final

When declaring a case object, make it final.


While case objects are effectively final in Scala - you can’t extend them - they are not always flagged as such in the compiled bytecode.

Take the following code, for example:

class Test {
  case object foo

When foo is not flagged as final, running the class file through javap -c yields the following (truncated) output:

public class Test$foo$ implements scala.Product,scala.Serializable {
 // [...]

When it’s flagged as final, however, we get:

public final class Test$foo$ implements scala.Product,scala.Serializable {
  // [...]

To make matters worse, this is not even coherent accross Scala versions - the previous output was obtained with 2.12.7, but 2.11.8, for example, will compile both versions of as final.

This might not seem like a big deal - as far as scalac is concerned, is final either way and you won’t be able to extend it - not having this information in the bytecode will:

Checked by

This rule is not checked by any linter that I know of.