Doubt in K&B SCJP 5: Chap 2, SELF TEST, Q 11

[kunal dabir]

Hi,

Please refer to the 11th Question in Self Test of Chapter 2:

class Uber {
static int y = 2;
Uber(int x) { this(); y = y + 2; }
Uber() { y++; }
}
class Minor {
Minor() { super(y); y = y + 3; }
public static void main(String[] args) {
new Minor();
System.out.println(y);
}
}

Shouldn't the Answer be 'Compilation Error', as 'y' is a static variable and hence wont be inherited in the 'Minor' class which is using it in the default constructor?

[Sam Aran]

Originally posted by kunal dabir:
Hi,

Please refer to the 11th Question in Self Test of Chapter 2:

class Uber {
static int y = 2;
Uber(int x) { this(); y = y + 2; }
Uber() { y++; }
}
class Minor {
Minor() { super(y); y = y + 3; }
public static void main(String[] args) {
new Minor();
System.out.println(y);
}
}

Shouldn't the Answer be 'Compilation Error', as 'y' is a static variable and hence wont be inherited in the 'Minor' class which is using it in the default constructor?

I'm assuming there's actually an "extends" keyword for Minor, since you're implying Minor extends Uber.  
In the case of Minor extending Uber, y *is* inherited...sort of.  It's the same trick that you can use with a Uber instance: if you have a new Uber (say, "u"), you can refer to y (in the same package) by u.y, even though "u" (I can't believe I have to put this in quotation marks, that is such crap!  I'm going to post everything in code tags from now on!) is not the class but an instance of the class.  
Since Minor is an instance of the class Uber, it actually does this with the implicit this.  
So, in a nutshell, it is "this.y"  which then translates to "Uber.y".  This does *not* result in a compilation error or warning in this case, because the static constant is from the class hierarchy.  
The warning would occur if you did use "new Uber().y" because in that case it is a "Has-A" relationship and not an "Is-A" relationship, so the compiler would complain that you should really be using Uber.y and refer to the class.  
Because Minor can be considered to be the class Uber, this warning is waived.

[ September 08, 2008: Message edited by: Samus Aran ]

[kunal dabir]

thanks Sam for your kind reply. I am sorry for missing the 'extends Uber'. If it missed 'extends', it would not anyhow work.

What i used to think was, static members declared in parent class cant be referred from child class. thats why i got confused. After reading your reply, i wrote this

public class StaticsInheritance {
public static void main(String[] args) {
Child.display();
Parent.display();
}
}
class Child extends Parent{
static void display(){
System.out.println(x=x+1);
}
}
class Parent {
static int x = 2;
static void display(){
System.out.println(x);
}
}

which prints 3 twice. It shows that x is still only in Parent, while Child updates Parent's copy.

Thanks again.