language agnostic - Can you explain this thing about encapsulation?

Question

In response to What is your longest-held programming assumption that turned out to be incorrect? question, one of the wrong assumptions was:

That private member variables were private to the instance and not the class.

(Link)

I couldn't catch what he's talking about, can anyone explain what is the wrong/right about that with an example?

Answer 1

public class Example {
  private int a;

  public int getOtherA(Example other) {
    return other.a;
  }
}

Like this. As you can see private doesn't protect the instance member from being accessed by another instance.

BTW, this is not all bad as long as you are a bit careful. If private wouldn't work like in the above example, it would be cumbersome to write equals() and other such methods.