CODEWITHSUNDEEP
javainheritance
barbara
barbara

Reputation: 3201

How inheritance works indeed?

Please help me to understand how inheritance works.

If I have two classes, parent and child. When I create an instance of child is parent class instance constructed as well or not?

My code of Parent class.

public class Parent {
    private char s;

    @Override
    public int hashCode() {
        return s;
    }
}

And Child

public class Child extends Parent {
    private int i;

    public Child(int i) {
        super();
        this.i = i;
    }

    @Override
    public int hashCode() {
        return i;
    }
}

And finally the test

public class Main {
    public static void main(String[] args) {
        Child child = new Child(100);
        System.out.println(child.hashCode());
        System.out.println(child.getClass().getSuperclass().hashCode());
    }
}

In output I get 

100
2000502626

So the hashes of objects are different. It means that when I create instance of Child it is also created instance of Parent. Am I right?

Upvotes: 0

Views: 115

Answers (5)

TheLostMind
TheLostMind

Reputation: 36304

Actually, there will be just one child object created. Since every child is a parent, the parent constructor will be invoked. if you print this in both child as well as parent instance methods, it will print the same (child object) check - this question

Upvotes: 1

Michał Schielmann
Michał Schielmann

Reputation: 1382

When you create a Child object a Parent constructor is invoked as well, because a Child is a Parent.

But when you do this: 

System.out.println(child.getClass().getSuperclass().hashCode());

you're not invoking Parents instance hashode. You are invoking hashCode() of the instance of the Class object. See what child.getClass().getSuperclass() returns. It returns an instance of type Class not of type Parent/Child. You cannot invoke Parents instance methods using child.getClass().getSuperClass() - that doesn't return the instance of a type, but an object representing this type.

Try doing this in child method:

@Override
public int hashCode() {
    System.out.println("In child hashCode: " + i);
    System.out.println("Parents hashCode: " + super.hashCode());
    return i;
}

This will return 100 and 0, as Parents s hasn't been initialized.

Upvotes: 3

Aakash Sigdel
Aakash Sigdel

Reputation: 9300

Yes. Both parent and child object are created. The child class constructor calls the parent(super) class constructor first, then only other functions of the child class are performed. As you can see from your own code two different values getting printed.

Upvotes: -1

Adi
Adi

Reputation: 2394

If I have two classes, parent and child, When I create an instance of child is parent class instance constructed as well or not?

Yes. It works through Constructor. When you call constructor of your child class to create object, it first calls its parent class contructor and hence creates object of parent class

Upvotes: 0

Kent
Kent

Reputation: 195059

Your question has nothing to do with inheritance.

the 100 you get from child instance's hashcode() method, as you expected.

The 2000502626 was from Parent.class, not Parent object.

Parent.class has type java.lang.Class

parent object has type Parent

Upvotes: 10

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