受保护的构造函数和可访问性 [英] Protected constructor and accessibility

问题描述

为什么我们不能使用受保护的构造函数来实例化一个类，如果它的子类在不同的包中？如果可以访问受保护的变量和方法，为什么不适用于受保护的构造函数？

Why can't we instantiate a class with a protected constructor if its child is in a different package? If protected variables and methods can be accessed, why doesn't the same rule also apply for a protected constructor?

pack1：

package pack1;

public class A {
    private int a;
    protected int b;
    public int c;

    protected A() {    
        a = 10;
        b = 20;
        c = 30;
    }
}

pack2：

package pack2;

import pack1.A;

class B extends A {
    public void test() {
        A obj = new A(); // gives compilation error; why?
        //System.out.println("print private not possible :" + a);
        System.out.println("print protected possible :" + b);
        System.out.println("print public possible :" + c);
    }
}

class C {
    public static void main(String args[]) {
        A a = new A(); // gives compilation error; why?
        B b = new B();
        b.test();
    }
}

推荐答案

到Java规范（ https： //docs.oracle.com/javase/specs/jls/se8/html/jls-6.html#jls-6.6.2.2 ）

6.6.2.2。合格访问 protected 构造函数

让 C 其中声明了 protected 构造函数的类，并且 S 是其声明中使用 protected 构造函数。然后：

6.6.2.2. Qualified Access to a protected Constructor

Let C be the class in which a protected constructor is declared and let S be the innermost class in whose declaration the use of the protected constructor occurs. Then:

• 如果访问是通过超类构造函数调用 super（...） code>或限定超类构造函数调用 E.super（...），其中 E

如果访问是通过匿名类实例创建表达式 new C（...）{...} 或一个合格的匿名类实例创建表达式 E.new C（...）{...} ，其中 E 是主要表达式，则允许访问。

If the access is by an anonymous class instance creation expression new C(...){...}, or a qualified anonymous class instance creation expression E.new C(...){...}, where E is a Primary expression, then the access is permitted.

如果访问是通过一个简单的类实例创建表达式 new C（...）表达式 E.new C（...），其中 E 是 ，或者方法引用表达式 C :: new ，其中 C 是 ClassType 可以通过类实例创建表达式（不声明匿名类）或方法引用表达式访问 protected 构造函数

If the access is by a simple class instance creation expression new C(...), or a qualified class instance creation expression E.new C(...), where E is a Primary expression, or a method reference expression C :: new, where C is a ClassType, then the access is not permitted. A protected constructor can be accessed by a class instance creation expression (that does not declare an anonymous class) or a method reference expression only from within the package in which it is defined.

在您的情况下，从 B的构造函数中访问 A 从 B / code>通过调用 super（）。但是，使用新的访问不合法。

In your case, access to the protected constructor of A from B would be legal from a constructor of B through an invocation of super(). However, access using new is not legal.

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