Kotlin cannot access protected abstract method

Question

I have the following classes structure:

abstract class Abstr{
    protected abstract fun m()
}

class Child : Abstr(){
    private val subChild: Abstr = Child()

    override fun m() = subChild.m()// Error:(12, 18) Kotlin: Cannot access 'm': it is protected in 'Abstr'
}

I got an exception Kotlin: Cannot access 'm': it is protected in 'Abstr'

It little bit confusing, because the same structure is legal for java.

According to kotlin docs

• protected — visible inside this class only + visible in subclasses too;

Is it bug or expected behavior?

Answer 1

It is designed behavior

Protected modifier in Kotlin similar to Java, but has additional restrictions.

Protected in Java:

• Visible for inheritance
• Visible in package

Protected in Kotlin:

• Visible for inheritance

So, according to the code in question we cannot access protected method

class Child : Abstr(){
    private val subChild: Abstr = Child()

    override fun m() = subChild.m() //Trying to access not inherited method 
}

There is similar restriction in Java, when we trying to access protected member from another package:

// FILE: a/SuperClass.java
package a;
public class SuperClass {
    protected void superFunction() {}
}

// FILE: b/ChildClass.java
package b;
public class ChildClass extends SuperClass {
    void testFunction() {
        ((SuperClass) new ChildClass()).superFunction(); // ERROR: superFunction() has protected access in a.SuperClass
    }
}

There is answer in issue tracker from Kotlin team: https://youtrack.jetbrains.com/issue/KT-21048

Answer 2

The current behavior is by design.

By calling subChild.m() you're trying to access an instance of Abstr from outside the object, so protected access prevents you from doing this.

Let me show you a short example to clarify the case

    abstract class ParentCl {
        protected var num = 1
        protected open fun m(){
        }
    }
    
    class ChildCl : ParentCl() {
        private val a0 : ParentCl = ChildCl()
        override fun m() {
            super.m() // 1-st case
            num = 2 // 2-nd case            
            a0.m() // 3-rd case
        }
    }

1. You're calling the protected ParentCl's fun from the child class. It will work fine.
2. You're modifying of protected variable from the child class. It will work fine.
3. You're calling the protected fun outside the context of the child class. This will not work.

Depends on what was your goal there are two solutions:

1. If you wanted to call m() from ParentCl you need to change the visibility from protected to internal or public.
2. If you wanted to call m() from the child class you need to declare the variable without the explicit type of its parent: private val subChild = Child().

Note: in case you will use m() from other children of ParentCl you need to enlarge the visibility scope inside child class: public override fun m() {...}