Why it prints this and not that?

Question

Consider this:

class Animal {
    Animal(){
        System.out.println("Animal's constructor called");
        this.indentifyMyself();//??????????????
        System.out.println("Exit Animal's constructor");
    }
    void indentifyMyself(){
        System.out.println("I'm an Animal");
    }
}
class Human extends Animal{
    Human(){
        System.out.println("Human's constructor called");
        super.indentifyMyself();
        System.out.println("Exit Human's constructor");
    }
    @Override
    void indentifyMyself(){
        System.out.println("I'm Human");
    }
}
public class Main {

    public static void main(String[] args) {
        new Human();

    }
}

Why does it print?

Animal's constructor called

I'm Human (??? I didn't expect this)

Exit Animal's constructor

Human's constructor called

I'm an Animal

Exit Human's constructor

And not:

Animal's constructor called

I'm an Animal

Exit Animal's constructor

Human's constructor called

I'm an Animal

If anyone can give a detailed explanation of this behavior, I will be very grateful. Please don't tell me redundant answers like "is a Polymorphic java behaviour". Thank in advance

Try to understand how inheritance works, and what runtime types of objects are. — banncee, Sep 05 '18 at 13:13
Related: https://stackoverflow.com/questions/23093470/java-order-of-initialization-and-instantiation — Mark Rotteveel, Sep 05 '18 at 13:19

score 2 · Accepted Answer · answered Sep 05 '18 at 13:11

2

The line

I'm Human

is printed instead of the expected

I'm an Animal

because the actual runtime type of the instance is Human and not Animal, hence the overridden function is chosen from the virtual method table.

As discussed here, the behaviour of Java and C++ differs in this aspect.

answered Sep 05 '18 at 13:11

Codor

17,447
9
29
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Thank, I can understand that and this is all about architecture. Yet is a bit confusing. As I understood when we create a derived class this is what happens: 1st initiates the base class and then the derived class. So when it calls the base class constructor (I thought that), in theory the Human instance does not exist yet, because it wasn't initiated at that point. So that's why I didn't aspect this behaviour. Thank – Litiec Sep 05 '18 at 14:14

score 0 · Answer 2 · edited Sep 06 '18 at 00:01

0

You have overridden that method and in run-time JVM executes that overridden new version of the method. Not the old one in Animal class

edited Sep 06 '18 at 00:01

Riki Raspoutine

27
6

answered Sep 05 '18 at 13:19

Randula Koralage

328
2
10

Why it prints this and not that?

Why does it print?

And not:

2 Answers2