Protected nested class not accessible to derived classes in other package

Question

Here is what I'm trying to accomplish

File 1: ./net/Class1.java

package net;

public class Class1
{
    protected static class Nested
    {

    }
}

File 2: ./com/Class2.java

package com;

import net.Class1;

public class Class2 extends Class1
{
    Nested nested = new Nested();
}

Here is the error I'm getting

>javac ./net/Class1.java ./com/Class2.java
.\com\Class2.java:7: error: Nested() has protected access in Nested
        Nested nested = new Nested();

Is this error expected? Am I doing something wrong?

Answer 1

Problem

Few important facts (which many people forget or are not aware of):

1. default constructors (including ones for static and non-static nested classes) have same visibility as visibility of class which they belong to. So in case of protected class Nested{...} its default constructor is also protected.
2. element with protected visibility can be accessed only from class which
   • belongs to same package as class which declared that element,
   • extends (explicitly or implicitly) class which declared it.

Your Class2 extends Class1 so it only have access to members of Class1 (including access to Nested type). But since it

• doesn't extend Nested (even implicitly, it only inherits access to it since it is protected)
• doesn't belong to same package as Nested

it can't access protected elements from Nested class (including constructors).

Solution:

To solve that problem make Nested constructor public by either

• explicitly creating no-argument constructor of Nested class with public modifier:

  package net;
  
  public class Class1 {
  
      protected static class Nested {
  
          public Nested(){
        //^^^^^^
          }
  
      }
  
  }
  

• making Nested class public (its default constructor will also become public - see point 1.)

  package net;
  
  public class Class1 {
  
      public static class Nested {
    //^^^^^^
      }
  
  }