java - 为什么 protected 静态字段在 Java 的不同类中可见

Question

下面是代码:

package ab:
public class A {
    protected static int var = 10;
    protected int var2 = 20;
}

和

    package cd;
    public class C extends A {
        A test;

        public C(){
            test = new A();
        }

        void printValues(){
            System.out.println(A.var); //this is perfectly visible
            System.out.println(test.var2); // here I get error saying var2 is not visible
        }
    }

我不明白为什么静态保护字段可以通过不同包中的 A 访问...

Answer 1

因为一个 protected 成员可以从任何包中的子类访问这一事实广为人知，我正在回答你问题的另一面:为什么 protected 实例字段对子类不可见？

像往常一样，寻找权威答案的地方是 Java 语言规范，在本例中为 Section 6.6.2 .我引用了那里找到的例子，因为它比前面的法律术语更容易理解。

TL;DR: 最好的理解方式是:protected 是一个内部继承类。从其所有子类的角度来看，A.var2 的行为类似于每个子类单独 的私有(private)成员，而不是父类(super class)的成员。所有这一切都已到位，因为 protected 旨在用于为扩展而设计的类中，以便子类可以访问那些被认为是扩展类的公共(public) API 的部分，但不能用于客户端类(class)的。

为了完成您的示例系列，我提交了另外两个示例:

System.out.println(this.var2);      // works---intended use of protected
System.out.println(((A)this).var2); // fails same as your test.var2

深思:)

Example 6.6.2-1. Access to protected Fields, Methods, and Constructors

Consider this example, where the points package declares:

package points;
public class Point {
  protected int x, y;
  void warp(threePoint.Point3d a) {
    if (a.z > 0)  // compile-time error: cannot access a.z
      a.delta(this);
  }
}

and the threePoint package declares:

package threePoint;
import points.Point;
public class Point3d extends Point {
  protected int z;
  public void delta(Point p) {
    p.x += this.x;  // compile-time error: cannot access p.x
    p.y += this.y;  // compile-time error: cannot access p.y
  }
  public void delta3d(Point3d q) {
    q.x += this.x;
    q.y += this.y;
    q.z += this.z;
  }
}

A compile-time error occurs in the method delta here: it cannot access the protected members x and y of its parameter p, because while Point3d (the class in which the references to fields x and y occur) is a subclass of Point (the class in which x and y are declared), it is not involved in the implementation of a Point (the type of the parameter p). The method delta3d can access the protected members of its parameter q, because the class Point3d is a subclass of Point and is involved in the implementation of a Point3d.

The method delta could try to cast (§5.5, §15.16) its parameter to be a Point3d, but this cast would fail, causing an exception, if the class of p at run time were not Point3d.

A compile-time error also occurs in the method warp: it cannot access the protected member z of its parameter a, because while the class Point (the class in which the reference to field z occurs) is involved in the implementation of a Point3d (the type of the parameter a), it is not a subclass of Point3d (the class in which z is declared).