How to explain the following sentences about private[this] and variance?

Question

In Chapter 19 ,"Programming in scala 2nd edition",how to explain the bold sentences ?

object private members can be accessed only from within the object in which they are defined. It turns out that accesses to variables from the same object in which they are defined do not cause problems with variance. The intuitive explanation is that, in order to construct a case where variance would lead to type errors, you need to have a reference to a containing object that has a statically weaker type than the type the object was defined with. For accesses to object private values, however,this is impossible.

Answer 1

I think the most intuitive way to explain what Martin is trying to say is to look at arrays in Java. Arrays in Java are covariant but don't type check according to covariance rules. This means they explode at runtime instead of compile time:

abstract class Animal {}
class Girafee extends Animal {}
class Lion extends Animal {}

public class Foo {
    public static void main(String[] args) {
        Animal[] animals = new Girafee[10];
        animals[0] = new Lion();
    }
}

The fact that I can do this is because:

1. Java doesn't restrict this at compile time (due to a design decision)
2. I have a reference to the underlying array which allows me to manipulate it's internal values

This doesn't hold when talking about private fields of a class from the outside.

For example, assume the following class:

class Holder[+T](initialValue: Option[T]) {
  private[this] var value: Option[T] = initialValue
}

When creating an instance of Holder, I am not visible to it's internal fields, thus I cannot manipulate them directly like I did with the Java array. This way, the compiler makes sure they are protected, and each manipulation to the field will have to be through a method, where the type checker is strict and doesn't allow funky business.