Package protected alternative in Kotlin

Question

In Java, we have package protected (default) modifier for classes, which allows us to have many classes in a single package but exposes only a few and keep the logic encapsu

Answer 1

As @hotkeys points out, you can use the internal keyword in a module or you can put all classes that would otherwise belong in a package inside a single file, but sticking several classes in a file may be a questionable design decision.

For me, the package visibility is helpful for its documenting value. I want to know what public interface some package is presenting to the rest of the project, hide factory implementation classes and so on.

So even if it's possible to access package-private classes and methods in Java, I still choose to use the package modifier.

For this I created a project with a single annotation:

package com.mycompany.libraries.kotlinannotations;

import static java.lang.annotation.ElementType.CONSTRUCTOR;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.SOURCE;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

@Documented
@Retention(SOURCE)
@Target({ TYPE, METHOD, CONSTRUCTOR })
/**
 * Use in Kotlin code for documentation purposes. 
 * 
 * Whenever a Kotlin class or method is intended to be accesible at package level only.
 *
 */
public @interface PackagePrivate {

}

Then I can use this annotation in any Kotlin project.

The second step, which I haven't done yet, is creating a PMD rule to enforce this with maven (or any other build tool for that matter) and also be able to see violations of the rule in my IDE with the pmd plugin.

There no is full Kotlin support in pmd at this moment but it seems to be expected at some point.