同じクラス内のオブジェクトはどのようにしてプライベート フィールドにアクセスできるのでしょうか?

Accessing Private Fields of Objects in the Same Class

In Java, the private access modifier restricts field visibility to the class in which the field is declared. However, a common misconception is that this restriction applies to objects within the same class.

Consider the following example:

class Person {
  private BankAccount account;

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

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

Why is accessing person.account possible?

The private modifier enforces encapsulation to protect the internal state of an object from external modification. However, within the same class, objects share the same implementation details. They are aware of the class's private fields and methods.

As Artemix explains:

"The idea is that 'outer world' should not make changes to Person internal processes because Person implementation may change over time (and you would have to change the whole outer world to fix the differences in implementation - which is nearly impossible)."

Since objects within the same class share the same implementation knowledge, granting them access to private fields ensures that they can always access and manipulate the internal state of the class correctly. If the implementation changes, only the class code needs to be updated, eliminating the need to modify code outside the class.

Therefore, OOP design allows private fields to have class-level access rather than object-level access to facilitate encapsulation while maintaining consistency within the class's implementation.

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