关于java：为什么我不能在包外使用受保护的构造函数？

Why can't I use protected constructors outside the package?

本问题已经有最佳答案，请猛点这里访问。

为什么我不能在包外为这段代码使用受保护的构造函数：

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

JAVA

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

即使我已经扩展了类，为什么我会得到错误？请解释

编辑：

编译错误：

The constructor Example() is not visible

相关讨论

通常，protected意味着只能访问同一包中的子类或类。但是，以下是jls中的建设者规则：

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.

作为一个例子，这不会编译

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public class Example extends Exception {

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

但这确实

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public class Example extends Exception {

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

这个也是

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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！