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

Question

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

Answer 1

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!

Answer 2

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      } }