Why does a Java constructor have to be public or p

Question

The following is my ProtectedConstructor.java source code:

package protectCon;

public class ProtectedConstructor{
    public int nothing;
    ProtectedConstructor(){
        nothing = 0;
    }
}

And following is the UsingProtectedCon.java source:

package other;

import protectcon.ProtectedConstructor;

public class UsingProtectedCon extends ProtectedConstructor{   //**Line 4**
    public static void main(String... a) {  
    }
}

When I compile UsingProtectedCon.java, I get error at Line 4 shown above. It says that ProtectedConstructor() is not public ; so cannot be accessed outside package.

However, since my class is public, shouldn't I be able to extend it outside package. I am anyway not creating any instance of it.

Now, if I make the constructor of ProtectedConstructor class as public or protected then the code compiles fine with no error.

So then why is it necessary even for the constructor to be public or protected, and not just have default access?

Answer 1

to avoid all confusion do it like this. package protectCon;

public class ProtectedConstructor{
    public int nothing;
    public ProtectedConstructor(){
        nothing = 0;
    }
}


package other;

import protectCon.ProtectedConstructor;
public class UsingProtectedCon extends ProtectedConstructor{   //**Line 4**

    public UsingProtectedCon(){
      super();
      }


  public static void main(String... a){   
    }
}

Answer 2

JLS 6.6.7 answers your question. A subclass only access a protected members of its parent class, if it involves implementation of its parent. Therefore , you can not instantiate a parent object in a child class, if parent constructor is protected and it is in different package.Since the default constructor of the subclass would try to call parent class constructor ,you got this error.

See this SO Post for details

Answer 3

in your class ProtectedConstructor is defined with Package-Private access This means that outside the package its not seen even by the classes that extend from your ProtectedConstructor class

Define your constructor with 'protected' access modifier instead and you'll be done:

 package protectCon;

public class ProtectedConstructor{
    public int nothing;
    protected ProtectedConstructor(){
        nothing = 0;
    }
}

Answer 4

Your constructor is not public. Default scope is package-private.

Answer 5

If child will have constructor which is private in parent,we can make child's instance with that constructor and cast it to parent's type. So to prevent this java compiler is not allowing constructor to have constructor which is private in parent.

Answer 6

constructor is a member of class like field and method so access modifier applies to it in the same manner it apples to all the member of class

when you extend the class A in B , A's its default constructor will get called from B's constructor implicitly (if you don't call any of the overloaded constructor)