Abstract Class?

[sonir shah]

Given the following source code, which of the lines that are commented out may be reinserted without introducing errors?

abstract class Bang 
{
 //abstract void f();  //(0)
   final    void g(){}
 //final    void h(){} //(1)
   protected static int i;
   private int j;
}
final class BigBang extends Bang 
{
 //BigBang(int n) { m = n } //(2) 
   public static void main(String args[]) 
   {
      Bang mc = new BigBang();
   }
   void h(){}
 //void k(){ i++; } //(3)
 //void l(){ j++; } //(4)
    
   int m; 
}

Options :
1)final void h(){} //(1)
2)BigBang(int n){m=n} //(2)
3)void k(){i++} //(3)
4)void l(){j++} //(4)
5)abstract void f(); //(0)
Correct Answer is 3)void k(){i++}
Ok, I agree with the answer.
But in addition I even feel that option 5) is right, because even if we insert the line :"abstract void f() in the class Bang, it is valid because Bang has already been declared Abstract.
Please let me know whether I am right?

[Rob Ross]

You're right that it would be ok in class Bang, because as you pointed out, it has been declared abstract, and so the method
abstract void f();
could be uncommented.
However, if you look at the subclass of Bang, BigBang, you will see that there is no method named f(). So if you uncomment that line in Bang, you have an abstract method that is not implemented anywhere, so the compiler won't let you compile the BigBang class unless you mark it abstract. Since BingBang is already declared final in the example, and not abstract, you'd get a compiler error if you uncomment line #0.
Rob