Why Can I Access Private Fields of Another Object in the Same Class?

Understanding Class-Level Access for Private Fields

OOP principles enforce encapsulation by restricting access to private class members to prevent unauthorized modifications. However, in the following code, why is it possible to access the account field of another Person object despite its private visibility?

class Person {
    private BankAccount account;

    Person(BankAccount account) { this.account = account; }

    public Person someMethod(Person person) {
        // Why is accessing the private field possible?
        BankAccount a = person.account;
    }
}

Class-Level vs. Object-Level Access

Contrary to the expectations of object-level access, private fields in Java have class-level access. This design stems from the following reasons:

• Encapsulation:
  The primary goal of private modifiers is to ensure encapsulation by limiting access to class internals. Object-level access would defeat this purpose, as objects of the same class could arbitrarily modify each other's states.
• Implementation Independence:
  Class-level access allows the implementation of a class to change without affecting external interactions. If fields had object-level access, any changes in the internal structure of the class would require corresponding modifications in all referencing objects.
• Internal Awareness:
  In the code snippet, both Person objects have the same internal implementation, including the account field. By having class-level access, objects within the same class can assume knowledge of each other's internals and operate accordingly.

Conclusion:

Class-level access for private fields is an intentional design choice that strengthens encapsulation, ensures implementation independence, and facilitates interactions between objects within the same class. While it may seem counterintuitive, this design is essential for maintaining the integrity and flexibility of object-oriented programming.

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