Inheritance

[Herbert Maosa]

This is a question from Jaworski Quiz :
What is the output of the following program ?
----------------------------------------------------------------
class Question{
String s ="Outer";
public static void main(String[] args){
S2 s2 = new S2();
s2.display();
}
}
class S1{
String s ="S1";
void display(){
System.out.println(s);
}
}
class S2 extends S1 {
String s ="S2";
}
A.S1
B.S2
C.null
D.S1S2

I thought the program would output S2, by using the inherited dispaly method from S1 to display its won s variable, which I thought hides the s variable in S1.Apparently the correct answer is, A, the display method displays the s variable of the S1 class. I dont understand. Anyone can educate me.
Thanks,
Herbert.

[Ajith Kallambella]

Herbert,
Observe that S2 does not override the display method in S1. This makes the call to S2Object.display() and variable reference 's' to get resolved at compile time.
One related point to note here ( for the exam ) is that variables are not polymorphic. They are always bound at compile time.
Hope this helps.
Ajith

[Anonymous]

Just to add to what Ajith has explained - All the variables are statistically bound and evaluated at the compile time where as all the methods barring private and static are dynamically bound and they are resolved at the run time.
I have modified code and added highlighted lines in the code given above.
class Question{
String s ="Outer";
public static void main(String[] args){
S2 s2 = new S2();
s2.display();
}
}
class S1{
String s ="S1";
void display(){
System.out.println(s);
}
}
class S2 extends S1 {
String s ="S2";
void display(){
System.out.println(s);
}
}
Now the output will be S2.
Hope this helps !!
Regards,
Milind

[This message has been edited by Milind (edited May 30, 2000).]

[Herbert Maosa]

Thanks Ajith and Milind.
Herbert.

[Greg Whelan]

While playing around with Herbert's code, I hit a mutation that I thought exhibited some very odd behavior. I'll include my code here for the curious (hint: there's one key concept that the behavior hinges upon)...

public class DiabolicalInheritance {
    public static void main(String[] args){
//S1 s1 = new S1();
//s1.display();        // no-can-do. display is private
System.out.println("Next:");
S2 s2 = new S2();
s2.display();
System.out.println("Next:");
S3 s3 = new S3();
s3.display();
    }
}
class S1{
    String s ="S1";
    private void display(){
System.out.println(s + " " + getBar());
    }
    String getBar() {
return "one";
    }
    void foo(){
display();
    }
} 
class S2 extends S1 {
    String s ="S2";
    String getBar() {
return "two";
    }
    void display(){
foo();
    }
}
class S3 extends S2 {
    String s ="S3";
    String getBar() {
return "three";
    }
    void foo(){
display();
    }
}

[Edward Man]

Let me expand the clue a little bit.
Any method that calls S1.display() method is statically binded to it because it is a private method.