New Doubt: from SCJP6.0 K&B MockExam-1

[Trivikram Kamat]

Just read a Topic on a Doubt from SCJP6.0 K&B Mock Exam

I think that Line 12 should give a compile Error "numbers has private access in Numbers"

negatives.numbers.add(-n);

Although negate() function is inside Numbers class, the negatives object is a new one.
And numbers.add(-n) is a direct access, which should be allowed only within methods of numbers object, and not this object.

The following code gives compile error, as private member numbers is accessed directly:

class NumbersTest {
	public static void main(String[] args){
        Numbers n = new Numbers();
		n.numbers.add(2);
   }
}

[Trivikram Kamat]

Just checked in C++, it behaves the same way

This code compiles properly:

#include<iostream>

class Numbers {
  private:
	int data;
  public:
	void function() {
		Numbers *num = new Numbers();
		num->data = 2;
	}
};

int main() {
	return 0;
}

But this one gives compiler error 'int Numbers::data' is private within this context

#include<iostream>

class Numbers {
  private:
	int data;
  public:
	void function() {
		
	}
};

int main() {
	Numbers *num = new Numbers();
	num->data = 2;
	return 0;
}

Can someone please explain how the access modifiers work in this case (in Java).

[Abimaran Kugathasan]

Private means in Java, that private members can be accessed in the same Class, not from other class. This is way, you got Error in your codes. You are accessing a private member from another class. Private isn't private for same class instances in Java, that's the previous code works.

[Trivikram Kamat]

Thanks Abhimaran for the explanation. So, Private members are private to the class and not the specific objects.
This is one way using which objects of same class can access private members of each other.

After this, I wrote some test codes for checking protected access modifier.
Protected members can be directly accessed through objects of Base class from the derived classes within the same package.
But it doesn't work with subclasses in different package, where one needs to use Derived class object to access base class protected members.

[Abimaran Kugathasan]

Trivikram Kamat wrote:
But it doesn't work with subclasses in different package, where one needs to use Derived class object to access base class protected members.

Could you please post the code here?

[Trivikram Kamat]

Consider Class ClassA from package A:

package A;

public class ClassA {
	protected int data;
}

And class ClassB from package B:

package B;
import A.ClassA;

class ClassB extends ClassA {

	void function() {
		ClassA obj = new ClassA();
		obj.data = 2;
	}
}

Compiling ClassB gives compile time error data has protected access in A.ClassA

But derived class ClassC from package A doesn't give the error:

package A;

class ClassC extends ClassA {

	void function() {
		ClassA obj = new ClassA();
		obj.data = 2;
	}
}

This is because protected members are accessible in kids + subclasses, as given in K&B
But in subclasses from different packages, they're accessible only through subclass object, and not through superclass object.

ClassB compiles fine, if following changes are done on Line 8:

ClassB obj = new ClassB();

This is just the way I understood the concept.
No questions about anything...

[Abimaran Kugathasan]

Yea, protected members can be accessed only through inheritance in the case, the sub class is in different package! Within the sane package, protected means default access level.

[Trivikram Kamat]

Thanks to Javaranch I understood the access modifiers fully
Otherwise, I would have easily made silly mistake while answering SCJP6