protected variable access

[naveen yadav]

Look a code :

package som;
public class Bed
{
public static int stc = 7;
public int ins = 3;
protected int pro = 99;
public static Bed ref = new Bed();
}

Following class tries to access the protected variable through the Bed class.

import som.Bed;

public class UserBed extends Bed
{

public static void main(String... arg)
{

System.out.println(ref.pro);      //Line 1        
System.out.println(new Bed().pro);       //line 2

}
}

points of my logic :
Bed class is visible in UserBed.
A protected variable can be accessed in different package by a subclass.

then why i cannot access protected variable ins through the object or reference of Bed class in UserBed class.




Regards
naveen

[Seetharaman Venkatasamy]

From JLS:

6.6.2.1 Access to a protected Member
Let C be the class in which a protected member m is declared. Access is permitted only within the body of a subclass S of C. In addition, if Id denotes an instance field or instance method, then:

If the access is by a qualified name Q.Id, where Q is an ExpressionName, then the access is permitted if and only if the type of the expression Q is S or a subclass of S.
If the access is by a field access expression E.Id, where E is a Primary expression, or by a method invocation expression E.Id(. . .), where E is a Primary expression, then the access is permitted if and only if the type of E is S or a subclass of S.

For more detail : http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.6.2

[Amu Hlongwane]

The moment you say new Super class inside the Child class you defeating the benefits of inheritance, which 1 of them is to be able to access Super class variables and methods including those marked protected. I'd advise you to use this and super, or call directly without using a reference varible.

[dennis deems]

Protected does not mean that any subclass instance can access the protected member of any superclass instance. It means that a subclass instance is allowed access to the protected member contained in itself or another subclass instance.

So you are allowed to do this:

public static void main( String[] args )
{
	UserBed ub = new UserBed();
	System.out.println( ub.pro );
}

And you are allowed to do this:

class WaterBed extends UserBed{ }

... 
// in class UserBed
public static void main( String[] args )
{
	WaterBed wb = new WaterBed ();
	System.out.println( wb.pro );
}

But NOT this:

class Futon extends Bed{ }

... 
// in class UserBed
public static void main( String[] args )
{
	Futon f = new Futon();
	System.out.println( f.pro );
}

[Amu Hlongwane]

But Protected DOES mean that any subclass instance can access the protected member of any super class.

[Dan Drillich]

Dennis Deems wrote:Protected does not mean that any subclass instance can access the protected member of any superclass instance. It means that a subclass instance is allowed access to the protected member contained in itself or another subclass instance.

Right, makes perfect sense. So, the following works just fine -

public class UserBed extends Bed  
{  
	int p = super.pro;

Regards,
Dan

[Dan Drillich]

Ok, this explains it perfectly -

Seetharaman Venkatasamy wrote:From JLS:

Let C be the class in which a protected member m is declared. Access is permitted only within the body of a subclass S of C.

Regards,
Dan

[Javin Paul]

Indeed, Plus one