java - Why is it allowed to access a private field of another object?

Question

Recently, I observed an unexpected behavior of accessing priavte fields in Java. Consider the following example, which illustrates the behavior:

public class A {

    private int i;  <-- private field!

    public A(int i) {
        this.i = i;
    }

    public void foo(A a) {
        System.out.println(this.i);  // 1. Accessing the own private field: good
        System.out.println(a.i);     // 2. Accessing private field of another object!
    }

    public static void main(String[] args) {
        (new A(5)).foo(new A(2));
    }
}

Why I am allowed to access the private field of another object of class A within the foo method (2nd case)?

Answer 1

Private fields protect a class, not an instance. The main purpose is to allow a class to be implemented independently of its API. Isolating instances between themselves, or protecting the instance's code from the static code of the same class would bring nothing.