Why can't I use protected constructors outside

问题:

This question already has an answer here:

• What is the difference between public, protected, package-private and private in Java? 24 answers

Why can't I use protected constructors outside the package for this piece of code:

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

Check.java

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

1. Why do i get the error even though i have extended the class? Please explain

EDIT:

Compilation error:

The constructor Example() is not visible

回答1:

protected modifier is used only with in the package and in sub-classes outside the package. When you create a object using Example ex=new Example(); it will call parent class constructor by default.

As parent class constructor being protected you are getting a compile time error. You need to call the protected constructor according to JSL 6.6.2.2 as shown below in example 2.

package Super;

public class SuperConstructorCall {

    protected SuperConstructorCall() {
    }

}

package Child;

import Super.SuperConstructorCall;

public class ChildCall extends SuperConstructorCall
{

    public static void main(String[] args) {

        SuperConstructorCall s = new SuperConstructorCall(); // Compile time error saying SuperConstructorCall() has protected access in SuperConstructorCall
    }
}

Example 2 conforming to JLS 6.6.2.2:

package Super;

    public class SuperConstructorCall {

    protected SuperConstructorCall() {
    }

}

package Child;

import Super.SuperConstructorCall;

public class ChildCall extends SuperConstructorCall
{

    public static void main(String[] args) {

        SuperConstructorCall s = new SuperConstructorCall(){}; // This will work as the access is by an anonymous class instance creation expression 
    }
}

回答2:

Usually protected means only accessible to subclasses or classes in the same package. However here are the rules for constructors from the JLS:

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.

As an example, this does not compile

public class Example extends Exception {

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

but this does

public class Example extends Exception {

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

and so does this

public class Example {

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

So the rules are clear, but I can't say I understand the reasons behind them!

回答3:

In fact you are already using protected constructor of Example because Check has an implicit constructor and implicit Example constructor call:

public Check() {
    super();
}