need help from JQ+

[Ashik Uzzaman]

I just gave a Test exam of JQ+ and found this queestion.
Question ID :957672686580
Consider following two classes:
//in file A.java
package p1;
public class A
{
protected int i = 10;
public int getI() { return i; }
}
//in file B.java
package p2;
import p1.*;
public class B extends p1.A
{
public void process(A a)
{
a.i = a.i*2;
}
public static void main(String[] args)
{
A a = new B();
B b = new B();
b.process(a);
System.out.println( a.getI() );
}
}
What will be the output of compiling and running class B ?
I thought it will print but it does not compile. JQ+ is really a good exam simulator! I have read the answer but need further clarifications....

------------------
azaman

[Gagan Indus]

Ashik
as u might hav tried as well.. the code wont compile ..and it shld not
the comipler compalins this way ;

B.java:19: i has protected access in p1.A
a.i = a.i*2;
^
B.java:19: i has protected access in p1.A
a.i = a.i*2;
^
2 errors

well this is exaclty wht is expected from this code
following r the ways to access a member wid protected access modifier ;
- Class itself can acess 'protected' member ( of course)
- Subclass( or other classes) in the same package can access the 'protected' memeber as well ( obvious enough )
- Subclass in another package can access 'protected' memeber , But there is a string-attached !
the catch is tht Subclass in another package should access protected-memeber either through its own reference
( like, expicitly using keyword this or tht can be implicit too )
or using reference of it subclass . No other way it shld try to access protected members
( and wht could be other ways?? just wht our problematic code tries to do is a OTHER-way ,
subclass-in-another-package canNOT use the reference of class itself, or its super class ( or super's super-clas etc )to access protected-member)

i quote a line from khalid's 4th chapter

...a subclass in another package can only access protected memebers in the superclass via references of its own type or a subclass...

well.. if we change the code a lill bit , then it goes-well with the access rules for protected modifier

//in file A.java
package p1; 
public class A
{
protected int i = 10;
public int getI() { return i; }
}
//in file B.java
package p2;
import p1.*;
public class B extends p1.A
{
public void process(A a)
{
i = i*2;               /// here is where changes are 
}
public static void main(String[] args)
{
A a = new B();
B b = new B();
b.process(a);
System.out.println( a.getI() );
System.out.println( b.getI() );  //more changes 
}
}

this code compiles n run wid output;
10
20
remember , protected modifier 'protects' access through a proper-reference type
for subclasses in diff package , refernce-type need to be out of these two, subclass-in another package or its subclasses
for more on this i refer u to 4th chapter of khalid ( page 117)
or sec 6.6.2 of JLS 2nd ed ( a similar example is 6.6.7 in JLS , must see)
and after going through all the refrences , n understanding the concept ,
give a look to following code ( i changed the above code ) , n if u hav got wht
protected is all about , this code shld be breeze for u

//in file A.java
package p1; 
public class A
{
protected int i = 10;
public int getI() { return i; }
}
//in file B.java
package p2;
import p1.*;
public class B extends p1.A
{
public void process()
{
int k = ((B)foo(false) ).i * 10 ;
int l= ((B)foo(true) ).i * 20 ;
System.out.println("k ="+k ,"l= "+l);
}
public static void main(String[] args)
{
B b = new B();
b.process();
}
public A foo(boolean b)
{
  if(b) return ( new A() );
  else return ( new B());
}
}

have chilling preparation ! n good luck !


[This message has been edited by Gagan Indus (edited August 20, 2001).]

[Ashik Uzzaman]

Thnx Gagan!
Though a little bit comlex example (code) , i actually knew the theory but realized it after getting ur strings from JLS. This is really Java.....
------------------
Muhammad Ashikuzzaman (Fahim)
When you learn something, learn it by heart!