java - 为什么我不能在包外使用 protected 构造函数？

Question

这个问题在这里已经有了答案:

What is the difference between public, protected, package-private and private in Java? (30 个回答)

关闭 7 年前。

为什么我不能为这段代码在包外使用 protected 构造函数:

package code;
public class Example{
    protected Example(){}
    ...
}

检查.java

package test;
public class Check extends Example {
  void m1() {
     Example ex=new Example(); //compilation error
  }
}

1. 为什么即使我已经扩展了类(class)，我也会收到错误消息？请解释

编辑:

编译错误:

The constructor Example() is not visible

Answer 1

通常 protected 意味着只能被同一包中的子类或类访问。然而，这里是来自 JLS 的构造函数的规则:

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:

If the access is by a superclass constructor invocation super(...), or a qualified superclass constructor invocation E.super(...), where E is a Primary expression, then the access is permitted.

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.

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.

例如，这不会编译

public class Example extends Exception {

    void method() {
        Exception e = new Exception("Hello", null, false, false);
    }
}

但这确实

public class Example extends Exception {

    Example() {
        super("Hello", null, false, false);
    }
}

这也是

public class Example {

    void method() {
        Exception e = new Exception("Hello", null, false, false) {};
    }
}

所以规则是明确的，但我不能说我理解它们背后的原因!