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constructor invocation-doubt

Vidya Ramachandran
Greenhorn

Joined: Nov 04, 2005
Posts: 24
how does the following program work?

class Base {
int i;
Base() { add(1); }
void add(int v) { i += v; }
void print() { System.out.println(i); }
}
class Extension extends Base {
Extension() { add(2); }
void add(int v) { i += v*2; }
}
public class Qd073 {
public static void main(String[] args) {
bogo(new Extension());
}
static void bogo(Base b) {
b.add(8);
b.print();
}
}


the answer is 22

can someone plz explain
Sreeram Desigan
Greenhorn

Joined: Apr 07, 2006
Posts: 23
Dear Smarty
The line bogo(new Extension()) first creates a new Extension object before calling the bogo method. The Base class constructor of Extension class will execute first. Thus the constructor Base() will get executed. The Base() calls add(1) method. Now this call can be interpretted as this.add() sice the 'this' is an object of derived class its add method gets called ( polymorphism). So i will be set to 2. then derived class constructor is called so Extension(2) will be called. This causes i to be 6. Then in bogo method v have b.add(8) which causes i to become 22.
Please let me know if further clarification is needed.
aditya makam
Greenhorn

Joined: Mar 30, 2006
Posts: 9
Hi,

In the above code, you have to understand the overriding concept of methods.See the below example

For the above program,it will print derived three times.It will always call the print() in derived class,bcoz you have overridden it.

In the same way for the program which you gave ,at the statement

bogo(new Extension()); This will goto constructor of Extension and then u can assume super(); by default ,so it will goto base class constructor now it will call add(1),then the method in Extension class will be called.
Now i value is 2. Again it will come back to Extension constructor ,now it will call add(2) ,now the method in Extension will be executed and i value becomes 6. Now the creation of the instance (new Extension()) is over. It will go into the method. Now b.add(8) will be called.Now also the method in Extension class will be called,bcoz you have overridden the method add().i value becomes 22.

I think this explaination might have helped u.If I am wrong,correct me.


Be Happy and Enjoy Life.
Mark Spritzler
ranger
Sheriff

Joined: Feb 05, 2001
Posts: 17249
    
    6

"smarty"

Change your display name back to your real first and last names.

Mark


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