why an instance member in super class cannot accessed by Super class reference in Subclass in another package

Question

I have a Super-class A in PackageOne and a Sub-class B which is in PackageTwo.

package PackageOne;

public class A {        
protected number = 0;        
}

Second Class is here

package PackageTwo;

public class B extends A {
    public static void main(String args[]) {
       A a = new A();
       System.out.println(a.number);// Here is the error occurs        
    }        
}

Error is The field Person.name is not visible;

As I know a protected member can be accessed by a super-class reference, but why the error occurs.

In this case `B` isn't even a subclass of `A`. Why would you *expect* to have access? (Also, please take more care when formatting your post in future. I've fixed it up this time, but look at the preview before you hit post.) — Jon Skeet, Jan 06 '16 at 13:14
Where is "The field Person.name"? Generally what doesn't exists isn't visible. — MikeCAT, Jan 06 '16 at 13:35
@MikeCAT OP obviously meant "The field **A.number** is not visible", but instead pasted the actual error instead of the error that occurs in his example. — Fred Porciúncula, Jan 06 '16 at 13:40

YoungHobbit · Answer 1 · 2016-01-06T13:39:15.103

2

Controlling Access to Members of a Class

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

Because it is protected member. You need to extend (inheritance) the class to access the protected members of a class in another package.

Instead of accessing via A access it via B. Once you extend the class A it becomes the part of class B. You cannot access it using the reference of A.

public class B extends A {
  public static void main(String args[]) {
    B b = new B();  // create the instance of B.
    System.out.println(b.number);  // access via the b.
  }
}

You are also missing the data type in class A:

public class A {
  protected int number = 0;
}

In the same package you can reference the members using the instance reference. Check the below code:

package PackageOne;

public class C {
  public static void main(String[] args) {
    A a = new A();
    System.out.println(a.number); // this works.
  }
}

edited Jan 06 '16 at 13:39

answered Jan 06 '16 at 13:15

YoungHobbit

13,254
9
50
73

2

This won't work if `A` and `B` are in different packages, as OP states in his question. – Fred Porciúncula Jan 06 '16 at 13:30
@ThiagoPorciúncula I got it. and updated the answer. – YoungHobbit Jan 06 '16 at 13:32
1

Now it works, even though that's a little different from OP's situation. – Fred Porciúncula Jan 06 '16 at 13:33
@ThiagoPorciúncula It is different, because what OP is trying to do is completely wrong and compilation error in java. – YoungHobbit Jan 06 '16 at 13:36
1

Yup. That's basically what I wrote in my answer. – Fred Porciúncula Jan 06 '16 at 13:38

Fred Porciúncula · Answer 2 · 2016-01-06T13:26:13.583

If you are in a different package, you can't access the field of another instance. You can only access the field within yourself.

Class A:

package a;

public class A {
    protected int i;
}

Class B:

package b;

public class B extends A {

    void m() {
        A a = new A();
        System.out.println(a.i); // error here!

        System.out.println(i);   // but no error here
    }
}

If A and B were in the same package, though, there would be no error whatsoever.

score 1 · Answer 3 · edited May 23 '17 at 12:31

Protected methods can only be accessible through inheritance in subclasses outside the package. And hence the second approach tryMeProtected(); works.

The code below wont compile because we are not calling the inherited version of protected method.

Example
Class1 c = new ClassA();
c.tryMeProtected(); // ERROR: tryMeProtected() has protected access in ClassA

Follow This stack link for more explaination.

score 0 · Answer 4 · edited May 23 '17 at 11:51

That's not how inheritance works. You don't make a class a subclass of another one by creating an instance of it.

You have to extend it.

public class B extends A

This will allow you to access protected fields. However, you will have to import PackageOne.A to do this.

EDIT

See this SO answer by @David Segonds on how these keywords work

            | Class | Package | Subclass | World
————————————+———————+—————————+——————————+———————
protected   |  y    |    y    |    y     |   n

You have to be a subclass or in the same package to access these properties.

From the official documentation:

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

@ThiagoPorciúncula Thanks for bringing it to my attention. See my edited answer — Arc676, Jan 06 '16 at 13:27

why an instance member in super class cannot accessed by Super class reference in Subclass in another package

4 Answers4