query on method over riding

[dipu menon]

I have a basic question on method overriding.
I have used 2 classes in different packages for illustrating this scenario.

Parent.java
-----------

package package1;
 
import package2.Child;
 
public abstract class Parent {
 
protected abstract void hello();
 
public static void main(String[] argStrings) {
Parent parent = new Child();
parent.hello();
}
}

Child.java
----------

package package2;
 
import package1.Parent;
 
public  class Child extends Parent{
 protected void hello()
 {
 System.out.println("hello from child");
 }
 
}

Since the child is in a different package, the protected method hello() in child is not visible to the Parent. But when I run the program, the hello() method in child is getting executed.
Does this means that, once a child class over rides the method, the child method would be always called irrespective of the access modifier?

[edit]Add code tags. CR[/edit]
[ September 28, 2008: Message edited by: Campbell Ritchie ]

[Abhi vijay]

The protected access modifier can be accessed by all classes in the same package as well as by the subclass,even if its in a different package.Moreover, in case of overridden methods, at runtime, the call to the method depends on the object type and not the refernce type.

So in this case, the parent reference variable has a reference type Parent and object type Child. So, it will invoke method in the Child class( as it is the object type).
[ September 28, 2008: Message edited by: Abhi vijay ]

[Seetharaman Venkatasamy]

Good Question by dipu menon ,


ANY UPDATE please ?

[Gamini Sirisena]

Abhi's explanation is good.

Take a look here to check a table on access modifiers.

[dipu menon]

Hi Abi,
Thanks for your prompt reply. The explanation you have given about the protected access modifier is absolutely right. That happens incase of inheritance.
In my case,the main method is creating an object of child and referencing with the parent variable. The hello()method of child is not invoked by inheritance. Let me re-write the code to make it more readable. I am introducing a class called RunClass to create the Child object. The RunClass and Parent are in the same package and the Child is in a different package.

RunClass.java
-------------

package package1;
import package2.Child;
public class RunClass {
public static void main(String[] argStrings) {
Parent parent = new Child();
parent.hello();
}
}

Parent.java
-----------

package package1;
public  class Parent {
protected void hello() {
System.out.println("hello from Parent");
}
}

Child.java
-----------

package package2;
import package1.Parent;
public class Child extends Parent {
protected void hello() {
System.out.println("hello from child");
}
}

Just glance through the modified code. The RunClass and Parent are in the same package. So a call to the "protected parent.hello()" inside RunClass would be OK for the compiler.
During runtime, the JVM sees that the parent reference variable is referencing a Child object, but the hello() method of the Child class should not be accessible to RunClass, which is in a different package.
I think you got my point.

[edit]Add code tags. CR[/edit]
[ September 28, 2008: Message edited by: Campbell Ritchie ]

[Abhi vijay]

Why do you think hello() method of the Child class should not be accessible to RunClass, which is in a different package???

[dipu menon]

because its protected.
If you insert the following code inside RunClass, You'll get compilation error.

Child child = new Child();
child.hello();


This is because, the hello() is protected and only subclasses of the Child
would have access to it.

[Campbell Ritchie]

Originally posted by dipu menon:
This is because, the hello() is protected and only subclasses of the Child
would have access to it.

No, it is because the RunClass class is in a different package.

The keyword "protected" allows access from the same package, or from subclasses of the class where "protected" is used.

[Seetharaman Venkatasamy]

Hi Cambpell,

but [RunClass not extends Child ]
------------------------------
RunClass parent = new Child();
parent.hello();
---------------------------
in this case how it is running because hello() is in protected which is in Child Class ,different package .more over it is not extends Child .

please clarify my doubt. i think Abhi gives the explanation for overridding and protected access specifier...

[Vijitha Kumara]

 Parent parent = new Child();   

for the compiler it checks the Parent reference and it's in the same package with RunClass so, that is allowed.