为什么允许访问另一个对象的私有字段？ [英] Why is it allowed to access a private field of another object?

问题描述

最近，我观察到了在Java中访问priavte字段的意外行为。请考虑以下示例，其中说明了行为：

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));
    }
}

为什么我被允许访问另一个人的私人领域 foo 方法中的类 A 的对象（第二种情况）？

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

推荐答案

私有字段保护类，而不是实例。主要目的是允许类独立于其API实现。在它们之间隔离实例，或者从同一类的静态代码中保护实例代码不会带来任何结果。

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.

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