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Author Protected Access Modifierrizwanamujawar Ranch Hand

Joined: Feb 24,2012Posts: 36

posted Wednesday, March 28, 2012 14:55:28

Friends Iam studying SCJP book...

Now this protected modifier section is killing and confusing me... Ok let me start...

I have a class Parent in one package,the Child class in another package inherits the protected

member... upto here its working fine...

Now SCJP books reads that.... "what if another class in same package as a Child class can access

the protected member of Parent through Child by instantiating the Child class...?

The answer given in the book was straight forward i.e,no.. They further told that the protected

member accessed by subclass will behave private to all other class from here onwards....

For example:A class Neighbour in the same package as that of Child",can access the protected

member of Parent through Child "...

SCJP book is atleast saying no but when i try to do it practically the answer is quiet different... A

suggestion where y understanding is going wrong...?.I have written down the code here which is working fine but voilating what My SCJP book is

telling...view plaincopy to clipboardprint?

1. package let;2. public class Parent3. {
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4. protected void hmmm()5. {6. System.out.println( "Iam protected" );7. }8. }9.

10. package kya;11. import let.Parent;

12. class Child extends Parent13. {14. public void test()15. {16. System.out.println( "Are you protected:" );17. this .hmmm();18. }19. }20. 21. 22. package kya;23. class Neighbour24. {25. public void letMeTry()26. {27. System.out.println( "Can i acsess the protected member through my friend Child...?"28. Child c= new Child();29. c.test();30. }31. 32. 33. public static void main(String args[])34. {35. Neighbour n= new Neighbour();36. n.letMeTry();37. }38. }

MohamedSanaulla Bartender

Joined: Sep 08,2007Posts: 2837

I like...

posted Wednesday, March 28, 2012 15:19:35

Did you try c.hmmm(); ? It says that accessing the protected method other than through

inheritance in a different package is not possible. But in the example you are accessing the

protected method in the instance of Child class, so you are still in the context of the Child class.

Mohamed Sanaulla | My Blog
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okay, here's the link: http://aspose.com/file-tools

subject: Protected Access Modifier

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protected member access protected modifier K&B Ch10#8 protected access question Doubt in protected modifier

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Author Doubt in protected modifier

ramya ray Ranch Hand

Joined: Aug 11,2006Posts: 101

posted Tuesday, September 12, 2006 02:25

It is mentioned that a subclass outside the package can not access member

which has protected modifier using a reference to an instance of

superclass..

But below code gives the different behaviour.

package fruit;

public class parent{

protected static void p1( ){

System.out.println("Parent class");}

}

import fruit.parent;

public class child extends parent{
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public static void main ( String ARGS[]){

child c = new child();

c.p1();

}}

class child complies and output : "parent class"

Please explain why it is not giving the compile error.

Thanks in advance.

Keith Lynn Ranch Hand



Here you are not accessing the method p1() with a superclass reference.

You are accessing it with a subclass reference.

[ September 11, 2006: Message edited by: Keith Lynn ]

Henry Wong authorSheriff


I like...


protected methods may be accessed from any class in the same

package and any subclass. Since child is a subclass, it may call the

protected method, even though it is not in the same package.

[Clarification: By "and", I did not mean to imply that subclass must be in

the same package. I meant that any class in the same package *and* any

subclass regardless of package location, may access the protected method.]

Henry

[ September 11, 2006: Message edited by: Henry Wong ]
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Books: Java Threads, 3rd Edition , Jini in a Nutshell , and Java Gems (contributor)

VaibhavChauhan Ranch Hand



if you want to challenge the quote that you mentioned, write:

view plaincopy to clipboardprint?

1. parent p= new child();2. p.p1();

this will not compile.

hope you got it.

[ September 12, 2006: Message edited by: Vaibhav Chauhan ]

Chetan Raju Ranch Hand



child c = new child();

c.p1();

please check which type c is.

I agree. Here's the link: http://ej-technologies/jprofiler

subject: Doubt in protected modifier

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Protected - access modifier

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package, Cannot access public variable

Sandra Bachan Ranch Hand


posted Wednesday, June 09, 2010 06:22:21

Hello,

I have two programs based on code from Sierra/Bates Chapter 1:

location: /one/src/Parent.java


1. package certification;

2. 3. public class Parent {4. public int x = 9 ;5. }

location: /one/src/Child.java


1. package certification;2. 3. class Child extends Parent {4. 5. public static void main (String[] args){6. 7. System.out.println( "x is " + x);8. }9. }

I navigate to one directory and type in javac -d ./classes/ ./src/Parent.java

./src/Child.java . Below is the error:

non-static variable x cannot be referenced from a static context

System.out.println("x is " + x);
http://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-package


9/46

One would think the code compiles and executes considering that Child inherits from Parent,

Child is in same package as Parent, and the variable is PUBLIC.

ChristopheVerr Sheriff

Joined: Nov 24,2005Posts: 14563

I like...


The problem is that you are accessing the instance variable from the main method, which is a

static method. Read the error message again : non-static variable x cannot be referenced

from a static context . I don't know if you have already studies what static variables and static

methods are. Try to make a public non static method in Child, and try System.out.println("x is "

+ x); again.

[My Blog] All roads lead to JavaRanch

Sahil Kapoor Ranch Hand



Hello sandra Bachan , this is the most frequent mistake encountered by JAVA learners.

Just to follow one rule :

Non-Static members cannot be accessed from Static context ie Static methods directly without

referring to any object.

Reasoning :- Static members could exist before object creation.

So when you are trying to access variable x which is inherited from the parent class, it is still

non- static. You try to access a non-static variable from the static main method and hence the

error. Because at that time if suppose,say, 100 objects of the child class already exists on the
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10/46

heap, then compiler complains because it does not resolve that whose x are you trying to refer .

But But But if you say i am referring to object number ,say 75, then you would say ob75.x , now

compiler knows that you are referring to x which exists inside object75, and hence it is allowed.

SCJP 6.0 96%

(Connecting the Dots ....)

MartinVanyavchich Ranch Hand



All that said, you have two options to make that work

One way, make x static:


1. package certification;2. 3. public class Parent {4. public static int x = 9 ; // note the static modifier 5. }


1. package certification;2. 3. class Child extends Parent {4. 5. public static void main (String[] args){6. 7. System.out.println( "x is " + x);8. }9. }

Second way, instantiate Child or Parent and invoke the call on the object:view plaincopy to clipboardprint?

1. package certification;2. 3. public class Parent {4. public int x = 9 ;
http://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-packagehttp://www.coderanch.com/t/498532/java-programmer-SCJP/certification/Child-extends-Parent-Belongs-package


11/46

5. }


1. package certification;2. 3. class Child extends Parent {4. 5. public static void main (String[] args){6. Child child = new Child(); // would work with Parent class too7. 8. System.out.println( "childs x is " + child.x);9. }10. }

Hope this helps

SCJP 6, OCMJD 6, OCPJWSD 6I no good English.

Sandra Bachan Ranch Hand

Joined: Feb 18,

2010Posts: 431

posted Saturday, June 12, 2010 18:56:06

These are all very helpful and now my code works.Thanks guys!

@ Sahil: I really appreciate you including the reasoning, i.e. static members can exist before

object creation


subject: Child extends Parent, Belongs to same package, Cannot access public variable

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default vs protected access

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12/46

bet u cant ans this

Code from SCJP K&B Book won't compile

problem on extending a class declared on the same package!!




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13/46

Author default vs protected access

Gyan Shankar Ranch Hand

Joined: Dec 12,2005Posts: 65

posted Saturday, December 24, 2005 10:34

Why is default more restrictive than protected? this is what is read in books.

default members can be accessed via the reference.member but for

protected you cant do this.

Please explain

Check this

Parent.java view plaincopy to clipboardprint?

1. package cert;2. public class Parent3. {4. int a = 10 ;5. }

Child.java view plaincopy to clipboardprint?

1. package cert;2. 3. public class Child4. {5. public int useParent()6. {7. Parent p = new Parent();8. return p.a;9. }10. }

Use.java view plaincopy to clipboardprint?
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14/46

1. import cert.*;2. public class Use3. {4. public static void main(String[] args)5. {6. Child c = new Child();7. System.out.println(c.useParent());8. }9. }

This code wont work if int a = 10 is made protected.

I think default can do certin things which protected cant

also protected can do certain things which default cant

So why is it said default is more restrictive?

[ December 23, 2005: Message edited by: Jeff Kumar ]

[ December 23, 2005: Message edited by: Jeff Kumar ]

SCJP(1.4), SCWCD(1.4), SCBCD(1.3), SCDJWS

vipul patel Ranch Hand

Joined: Oct 16,2005Posts: 146


when you are saying default, it means that it is package level accecibility. it

is NOT available outside of the package. try it.

in contrast to that, protected is visible outside package also. But there is

important difference here.

protected has something to do in subclasses. so a protected member insuperclass is also visible in subclass of that defined in another package.

where as default is not.

I think you can try out different situations with some sample code. just

highlighting those.


15/46

(1) superclass in package A and subclass in package B. try default member

(2) superclass in package A and subclass in package B. try protected

member(3) try accessing a default member (defined in package A ) in package B.

(4) try accessing a protected member (defined in package A ) in package B.

I have my own understanding for above.

default is 'public' in same package and 'private' in all other packages.

protected is just protected regardless of any package.




I know what default means. only accessible in the same package

I am just asking why do boooks say protected is more accessible than

default even though as per my example shows that protected can only be

accessible to subclasses but default has no such restriction.

So... they are really not more accessible or more restrictive.

On certain situations default is more accessible but in other siutations

protected is more accessible.

Naresh Gunda Ranch Hand



This code wont work if int a = 10 is made protected


1. package cert;2. public class Parent3. {4. protected int a = 10 ;5. }
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16/46

Hi jeff,

Even if a has protected access in Parent class,the given program will work

and gives output 10.

Why is default more restrictive than protected?

Yes, default is more restrictive than protected.

'default' members can be accessed by the otherclasses in the same package

only.

'protected' members can be accessed by the otherclasses in the same

package

+

subclasses in any package.

Hence 'default' is more restricted than protected.




I have posted after trying the code

It wont even compile if a is made protected. Please try again.

You can only access a protected variable via inheritance and not via

reference variable.

Naresh Gunda Ranch Hand



Hi Jeff, I hv executed this program. Output 10

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17/46

1. 2. //Parent.java 3. package cert;4. public class Parent5. {6. protected int a = 10 ;7. }


1. //Child.java 2. package cert;3. public class Child4. {5. public int useParent()6. {

7. Parent p = new Parent();8. return p.a;9. }10. }


1. 2. //Use.java 3. import cert.Parent;4. import cert.Child;5. public class Use6. {7. public static void main(String[] args)8. {9. Child c = new Child();10. System.out.println(c.useParent());11. }12. }

Output: 10

You can only access a protected variable via inheritance and not via reference

variable
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18/46


19/46

protected class? Rules for Protected and Default Members






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AuthorRules for Protected and DefaultMembers
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20/46

Brian Reindel Greenhorn

Joined: Jan 04,2007Posts: 6

posted Friday, January 05, 2007 08:10

I am jotting down notes while studying for the SCJP exam, and I want to be

sure that I get this concept 100%. Would it be correct to make the following

two statements:

"A default member of a public class can only be accessed by a class or a

subclass within the same package."

"A protected member of a class can be inherited by a subclass in a different

package, but it cannot be referenced."

If this is not completely correct, or you have alternate ways of stating these

two rules, please let me know.

Thanks

marc weber Sheriff


I like...


Welcome to JavaRanch!

I'm moving this to our SCJP forum for special attention. (See links at top of

page to continue discussion there.)

"We're kind of on the level of crossword puzzle writers... And no one ever goesto them and gives them an award." ~Joe Strummer sscce.org

qunfeng wang Ranch Hand



I think you are right.

There is a good explanation in Ch1 of K&B's book.
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21/46

To be or not to be. I t's a question.

Priya Viswam Ranch Hand



>>"A protected member of a class can be inherited by a subclass in a

different package, but it cannot be referenced."

Protected member can be referenced in the subclass even if the subclass is

in a different package.

SCJP 1.5
SCWCD 1.4




Originally posted by Priya Viswam:

>>"A protected member of a class can be inherited by a subclass in a

different package, but it cannot be referenced."

Protected member can be referenced in the subclass even if the subclass is in

a different package.

Priya, if what you say is correct, then why in the book, does this cause an

error:


1. package certification;2. public class Parent {

3.

protected int x = 9 ;4. }5. 6. package other;7. import certification.Parent8. class Child extends Parent {9. public void testIt() {10. System.out.println(x); // no error - inheritence 11. Parent p = new Parent();12. System.out.println(p.x); // error - reference!
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22/46

13. }14. }

[ January 04, 2007: Message edited by: Brian Reindel ]

Priya Viswam Ranch Hand



The example shown by you is correct.

From the statement "A protected member of a class can be inherited by a

subclass in a different package, but it cannot be referenced.""

what i understood is that you can't access the protected variable in the

subclass.

If you are using a reference of the parent class then it is not inheritance.

That is the reason why you can't access that in your child class.

HarishParavasthu Greenhorn

Joined: Dec 21,

2006Posts: 23


Yes priya is right,Protected member in one package can be accssed in other

package only by inheritance and not by reference of suerclass...

harish

Harish Paravasthu




Originally posted by Harish Paravasthu:

Yes priya is right,Protected member in one package can be accssed in other

package only by inheritance and not by reference of suerclass...

harish


23/46

Which would mean that my original two statements are correct.

Thank you for everyone's response and for the assistance.

okay, here's the link: http://aspose.com/file-tools

subject: Rules for Protected and Default Members

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24/46

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AuthorIs Protected Method isaccessible?

Kiran ChandPanga

Ranch Hand


posted Friday, February 10, 2006 16:10

package testpkg.p1;

public class ParentUtil

{

public int x = 420;

protected int dostuff()

{

return x;

}

}
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25/46


26/46

subclasses even in different package) only.

Check this link to know more about accessbility.

http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html

Hope this helps.

Brindha.

Anju sethi Ranch Hand

Joined: Dec 26,2005

Posts: 91


Remember, protected access modifier always refers to .

that means this modifier is accessible to subclass outside package only thru

inheritence.

It is illegal to make an instance of superclass and access protected member

outside package as protected member would act as private or default to this

instance.

Also, protected member is inherited by derived class outside package and

not by any other subclass of the derived class.

it becomes private in the subclass outside package.

Please concentrate on rules of protected modifier outside package

thanks,
Anju Sethi

srilathakareddy Ranch Hand



hi Kiran Chand ,

here is the example to understand the concept

package testpkg.p2;

import testing.p1.ParentUtil;
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27/46

public class ChildUtil extends ParentUtil

{

public static void main(String[] agrs)

{ChildUtil child= new ChildUtil();

ParentUtil parent= new ParentUtil();

child.protectedMehtod();....we can do it

parent.protectedField; ...possible

parent.protectedMethod();///ERROR

parent.protectedField; //ERROR

}

}

this means through child class reference can have access to protected

members but not through parent class reference. This is the reason why we

use protected keyword


subject: Is Protected Method is accessible?

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Author protected access modifier

Puja S Ranch Hand


posted Sunday, January 23, 2005 02:56

Hi,

package testpkg.p1;

public class ParentUtil {

public int x = 420;

protected int doStuff() { return x; }
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}

package testpkg.p2;

import testpkg.p1.ParentUtil;public class ChildUtil extends ParentUtil {

public static void main ( String [] args) {

new ChildUtil().callStuff();

}

void callStuff() {

System.out.print("this " + this.doStuff() );

ParentUtil p = new ParentUtil();

System.out.print(" parent " + p.doStuff() );

}

}

which statement is true?

1. The code compiles and runs, with output this 420 parent 420.

2.If line 8 is removed, the code will compile and run.

3.If line 10 is removed, the code will compile and run.

4.Both lines 8 and 10 must be removed for the code to compile.

5.An exception is thrown at runtime.

The answer ios 3....Why? Can anybody explain me. I thought the

answer was 1.

Thanks.

MikeGershman

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30/46

Ranch Hand

Joined: Mar 13,2004Posts: 1272

In order to acess a protected member of a superclass that is in

another package, you must use a reference type of your class or

a subclass of your class. So this was OK, because it is of type

ChildUtil, but p is not OK because it is of type ParentUtil.

Mike GershmanSCJP 1.4, SCWCD in process

Puja S Ranch Hand



Thanks Mike,You said "You must use a reference of your class or a subclass of

your class to access the protected member".......I am not able to

understand how a subclass of the class can be used.Please can

you give me an example.

Thanks

Puja S Ranch HandJoined: Jan 06,2005Posts: 51


Thanks Mike........I understood the concept.

meena latha Ranch Hand


posted Tuesday, January 25, 2005 02:59

Hi ............i am clear about the concept can any one of you

please explain to me.

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JavaRanch Java Forums Professional Certification Programmer Certification(SCJP/OCPJP)

Author A ques's answer explanation not correct inK&B in chap 2

Amit Das Ranch Hand


posted Friday, April 29, 2005 01:31

hi all,

K&B chap 2 page 136 ques 6:

Given the following, view plaincopy to clipboardprint?

1. 1 . package testpkg.p1;2. 2 . public class ParentUtil {3. 3 . public int x = 420 ;4. 4 . protected int doStuff() { return x; }5. 5 . }6. 7. 1 . package testpkg.p2;8. 2 . import testpkg.p1.ParentUtil;9. 3 . public class ChildUtil extends ParentUtil {10. 4 . public static void main(String [] args) {11. 5 . new ChildUtil().callStuff();12. 6 . }13. 7 . void callStuff() {14. 8 . System.out.print( "this " + this .doStuff() );15. 9 . ParentUtil p = new ParentUtil();16. 10 . System.out.print( " parent " + p.doStuff() );17. 11 . }18. 12 . }

which statement is true?

A. The code compiles and runs, with output this 420 parent 420.

B. If line 8 is removed, the code will compile and run.

C. If line 10 is removed, the code will compile and run.

D. Both lines 8 and 10 must be removed for the code to compile.

E. An exception is thrown at runtime.
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33/46

well undoubtedly the answer is c as a subclass outside the pacakge of the superclass can access

protected member only by inheritance i.e. it can only access them by object reference of type th

class.

the explanation given says:


1. 1 . package testpkg.p1;2. 2 . public class ParentUtil {3. 3 . public int x = 420 ;4. 4 . protected int doStuff() { return x; }5. 5 . }6. 1 . package testpkg.p2;7. 2 . import testpkg.p1.ParentUtil;8. 3 . public class ChildUtil extends ParentUtil {9. 4 . public static void main(String [] args) {10. 5 . new ChildUtil().callStuff();11. 6 . }12. 7 . void callStuff() {13. 8 . System.out.print( "this " + this .doStuff() );14. 9 . [B]ParentUtil p = new ChildUtil(); //MAKE A NOTE[/B] 15. 10 . if ( p instanceof ChildUtil)16. 10 . System.out.print( " parent " + p.doStuff() ); //compilation erro17. 11 . }18. 12 . }

now if we go according to its explanation that

doStuff() can be accessed only by instances of the ChildUtil class

p should have accessed the protected member as its an instance of the class ChildClass.

in my opnion, the reference type used for such an acces should be of type subclass or its fu

subclasses.

plz verify my thought, i'll be deeply obliged.

thanx

amit
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34/46

[edited because unindented code is too painful to read - Jim]

[ April 28, 2005: Message edited by: Jim Yingst ]

Joe Sondow Ranch Hand

Joined: Apr 10,2005Posts: 195


You are right that their explanation was not totally complete. You showed a counterexample whe

doStuff() appears to be getting accessed by an instance of ChildUtil, but i t does not compile beca

is a ParentUtil reference. Since p.doStuff() is not a legal call from within ChildUtil, you get a com

error.

Here's the tricky part: the compiler doesn't know or care that p actually references a Chil

object . All the compiler knows or cares about is that p is a reference to a ParentUtil, and calling

doStuff() method on a ParentUtil is not legal from that point in the code. It looks like it sho

because of polymorphism , but actually polymorphism doesn't apply here because you're not allo

make the call p.doStuff() at all, regardless of what p actually references.

What I suspect the authors meant by "doStuff() can be accessed only by instances of the ChildUti

class" was that you would need an instance of ChildUtil that is being referenced by a

ChildUtil reference as well. They simply neglected to point out the case you presented, where th

reference type differs from the instance type.

[ April 28, 2005: Message edited by: Joe Sanowitz ]

SCJA 1.0 (98%), SCJP 1.4 (98%)

SaranyanNarayanan Ranch Hand



Joe could we say this:

When a protected member is accessed in a subclass using a parent class reference , it cannot let

polymorphic call compile as the protected member in the overriding class is private to the subcla

Joe Sondow Ranch Hand

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