java源码String类中anotherString.value.length这个怎么理解

看java源码时，不理解。value是String类的private属性，所以在本类中可以直接访问 value属性，

但是 anotherString.value，实例对象怎么也可以访问private属性，经过研究还是private访问修饰符理解不到位，在本类中，实例也可以访问本类的private属性

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    
    
    /** The value is used for character storage. */
    private final char value[];
    ...
    ...
    public boolean equals(Object anObject) {
    
    
        if (this == anObject) {
    
    
            return true;
        }
        if (anObject instanceof String) {
    
    
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
    
    
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
    
    
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }
}

举例验证

可以看到在A类中有 private a 属性，且在A类的test方法中，A的实例对象可以访问到 private a 属性

public class Test {
    
    
    static void main(String[] args) {
    
    
        A a1 = new A();
        B b1 = new A();
        a1.test(b1); // 3
    }
}

class B {
    
    

}

class A extends B {
    
    
  private String[] a = {
    
    "H", "E", "L"};

  public void test(B b) {
    
    
    A a = (A)b;
    System.out.println(a.a.length);
  }
}