Explain the reason for this output (Q37, Enthuware)

[s sivaraman]

public class Question37 extends Baap{

    /**
     * @param args the command line arguments
     */
    int h=44;
    int getH()
    {
        System.out.println("beta "+h);
        return h;
    }
    public static void main(String[] args) {
        // TODO code application logic here
        Baap b = new Question37();
        System.out.println(b.h + " "+ b.getH());
        Question37 q = (Question37)b;
        System.out.println(q.h + " "+ q.getH());
    }
    
}
class Baap
{
    int h=4;
    int getH()
    {
        System.out.println("baap "+h);
        return h;
    }
}

how come the output is
beta 44
4 44
beta 44
44 44

rather

4 beta 44
44
44 beta 44
44

[nick woodward]

This confused me too, way more than I thought it should have, so thanks a lot for the question!

Anyway I believe it's because the println in main has to evaluate the 'getH()' method before it can print anything, it's printing the concatenation, not each individual element. So "beta + h" in class Question37's method getH() preceeds main's System.out.println(b.h +""+ b.getH()).

I've a better explanation in more detail if you want it, but it helped me a lot to work it out, so it might help you too using the above info. I'm happy to give it to you if you want.

Regards,

Nick

[Lukasz Mazurek]

Isn't it the same as posted here: link

[Paul Anilprem]

and here: http://enthuware.com/forum/viewtopic.php?f=2&t=2075

[nick woodward]

might be worth marking questions like this as 'enthuware'?

it just popped up the next day in my standard test - i was pretty relieved to see a nice easy question

[Roel De Nijs]

nick woodward wrote:Anyway I believe it's because the println in main has to evaluate the 'getH()' method before it can print anything, it's printing the concatenation, not each individual element. So "beta + h" in class Question37's method getH() preceeds main's System.out.println(b.h +""+ b.getH()).

You are absolutely spot-on!

When this line of code is executed

System.out.println(b.h + " "+ b.getH());

this is the flow of execution:
1/ b.h is evaluated -> 4 (rule 1: which instance variable is accessed, is determined at compile time based on the type of the reference variable -> b is of type Baap)
2/ 4 and " " are concatenated -> "4 "
3/ b.getH() is evaluated -> getH() method in Question37 class is executed (rule 2: which method is executed, is determined at runtime based on the type of the actual object -> actual object is of type Question37)
3a/ the statement System.out.println("beta "+h); is executed
3a1/ h is evaluated -> 44
3a2/ "beta " and 44 are concatenated -> "beta 44"
3a3/ "beta 44" is printed
3b/ h is evaluated -> 44
3c/ 44 is returned
4/ "4 " and 44 are concatenated -> "4 44"
5/ "4 44" is printed
And that's why "beta 44" is in the output before "4 44".

This (very) detailed explanation lists 2 very simple (and hopefully easy to remember) but very, very, very important rules:

Which instance variables you can access is determined at compile time based on the reference variable type.

Which instance methods you can call/invoke is determined at compile time based on the reference variable type. Which instance method is actually executed is decided at runtime based on the type of the actual object (= polymorphism).

Hope it helps!
Kind regards,
Roel

[Roel De Nijs]

nick woodward wrote:might be worth marking questions like this as 'enthuware'?

Definitely! When you post a quote and/or a code snippet, you should always QuoteYourSources.

(Because it's already confirmed to be a question from Enthuware, I updated the subject accordingly)