Inheritance

[Nabila Mohammad]

import java.util.*;
class STest
{
 
   public static void main(String[] args) {
        bogo(new Extension());
    }
    static void bogo(Base b) {
        b.add(8);
        b.print();
    }
}
 

 
class Base {
    int i;
    Base() { add(1); }
    void add(int v) { i += v; System.out.println(i +  "in Base");}
    void print() { System.out.println(i); }
}
class Extension extends Base {
    Extension() { add(2); }
    void add(int v) { i += v*2; System.out.println(i +  "in 
 
Extension");}
}
 
    

The options are :

A.9
B.18
C.20
D.21
E.22


This is from Whizlabs.
I had opted for D. ie 21
But after running the program , i get the result E ie.22
It seems the add() method of Base class was never executed , when i ws expecting it to execute once (the first time)

Can some one explain why the Extension add() is executed the first time.

[Kevin Smither]

When the code is executing in the superclass constructor then the add method translates as this.add(1) since it is based on the current instance, and in the above case "this" is of type Exception and not Base.

[vidhya suvarna]

Kevin, i am also getting the value as 21.

here's how i got the value:
1) bogo(new Extension());-> invokes the constructor of Extension which invokes constructor of Base
Base() { add(1); } -> invokes add of base
void add(int v) { i += v; } -> i=0+1 (value of i is 1)

then return to constructor of Extension
Extension() { add(2); } -> invokes add of Extension
void add(int v) { i += v*2; --> i=1+2*2 (value of i is 5 now)

2) now execute bogo ()
bogo(new Extension())--> invokes with arg which a refernce of Extension
static void bogo(Base b) {
b.add(8); }
add() of Extension is invoked
i=i+v*2 i.e i=5+16= 21

can kindly let me know where i am going wrong in the code.

[Nabila Mohammad]

Did you run the program?

It gives the out put 22.

[anand sekaran]

hey i also get the output as 22. Could anyone make clear of its order of execution with clear explanation

[Ls chin]

Hi,

Why we get 22?

I think it's due to polymorphism but I'm not 100% sure.

My understanding is, when "bogo(new Extension())" is called, it will use the void add() that belongs to Extension class i.e. void add(int v) { i += v*2; } and it won't use the one in Base class which is void add(int v) { i += v; } because the Extension class method has overridden its parent/superclass method.

I hope that's the correct answer.

[Resh Joseph]

I tried to run the program as well, and interestingly, the add(int v) method of Extension class is being called twice before the b.add(8) statement, first with the value of int v as 1 and second time int v = 2. I could understand why it was 2, but puzzled at how it called the Extension class add(int v = 1) at first pass.
Could anyone please clarify?

[Resh Joseph]

I think Kevin has already given an answer for this. Sorry to bother.

[Nabila Mohammad]

Can some one please clarify this

[vidhya suvarna]

Nabila,
Hope this will help

1) bogo(new Extension());-> invokes the constructor of Extension which invokes constructor of Base
Base() { add(1); } -> invokes add of Extension
void add(int v) { i += v*2 } -> i=1*2 (value of i is 2)

then return to constructor of Extension
Extension() { add(2); } -> invokes add of Extension
void add(int v) { i += v*2; --> i=2+2*2 (value of i is 6 now)

2) now execute bogo ()
bogo(new Extension())--> invokes bogo with ref of Extension
static void bogo(Base b) {
b.add(8); }
add() of Extension is invoked
i=i+v*2 i.e i=6+16= 22

Only thing here that was causing problem was in (1) which add() will be invoked. But that will invoke add() of Extension. It is Method overridding and Object Type determines which method to be invoked at runtime.

[Ganeshkumar cheekati]

nice explanation vidhya.

[Christina Pong]

a very good question. Thanks for posting :-)