Can't access protected inner class while inheriting.

Question

Reading through "Thinking in Java" i stuck in ex:6 of Inner Classes chapter.

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Exercise 6: (2) Create an interface with at least one method, in its own package. Create a class in a separate package. Add a protected inner class that implements the interface. In a third package, inherit from your class and, inside a method, return an object of the protected inner class, upcasting to the interface during the return.

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This is my code:

IOne.java

interface

package intfpack;
public interface IOne{
     void f();
}

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COne.java

Class with protected inner class that implements the interface

package classpack;
import intfpack.*;
public class COne{
     protected class Inner implements IOne{
       public void f(){System.out.println("Inner class of COne");}
     } 
}

---

CTwo.java

Inheriting from class with protected inner class

package thirdpack;
import classpack.*;
import intfpack.*;

public class CTwo extends COne{
     public IOne getInner(){
       IOne io = new Inner(); 
       return io;
     }
     public static void main(String[] args){
       CTwo ct = new CTwo();
       ct.getInner();
     }
}

Copmiler says next:

javac CTwo.java
CTwo.java:9: Inner() has protected access in classpack.COne.Inner
       IOne io = new Inner(); 
                 ^
1 error

But the book says that i can access protected inner classes in derived class. Where is mistake?

Answer 1

The error message is complaining about the constructor being protected, not the class. But you haven't explicitly defined a constructor in the code you posted. In this case, according to the JLS, the default constructor will be protected (the same as the class).

Answer 2

You need to define a public constructor for the Inner class:

public class COne {

    protected class Inner implements IOne{

        public Inner() { }

        public void f(){System.out.println("Inner class of COne");}
    }
}

Answer 3

That is clear. But here is a really weird thing.

According to the JLS if CTwo extends COne.Inner it is expected to access Inner's protected constructor, but in practice it does not... See below.

package p1;
public class COne {
    public static class Inner {
        protected Inner() {}
    }
}

package p2;
public class CTwo extends COne.Inner {
    public void getIface() {
        new COne.Inner();
        // Compile time error anyway with the same complains:
        // "Inner() has protected access in p1.COne.Inner"
        // ...unlike my expectations...
    }
}