CODEWITHSUNDEEP
javaoop
peter Estifanos
peter Estifanos

Reputation: 151

Why does the reference have a size in the memory even if it doesn't refer to an object in this code example?

Here in class animal getObjectSize on variable 'name' return 0

class Animal {

    String name;

    public Animal() {
        System.out.println(ObjectSizeCalculator.getObjectSize(this.name));
    }
}

Here in class Dog getObjectSize on variable 'dogName' return 0

class Dog extends Animal{
    String dogName;

    public Dog() {
       System.out.println(ObjectSizeCalculator.getObjectSize(this.dogName));
    }

    public void dogplay(){
        System.out.println("dog playing");
    }
}

Here in class Main getObjectSize on Animal object return 16, but getObjectSize on Dog object return 24, but I thought it should return 16 just like animal object

public class Main {

    public static void main(String[] args) {

        Animal a = new Animal();
        Dog dog = new Dog();

        long objectSize = ObjectSizeCalculator.getObjectSize(a);
        long objectSize1 = ObjectSizeCalculator.getObjectSize(dog);

        System.out.println(objectSize+" "+objectSize1);

    }
}

What I think of is dogName reference size in memory equal 0, and name reference size in Animal class also equal 0, so why after creating a new object of Dog it has a larger size than animal object size?

When I remove (String dogName) from class dog, now Dog object size becomes equal to animal object size.

Upvotes: 0

Views: 148

Answers (1)

Mark Rotteveel
Mark Rotteveel

Reputation: 109146

The problem is that you are using an incorrect assumption. The use of

System.out.println(ObjectSizeCalculator.getObjectSize(this.name));

in your constructor does not get the size of the reference this.name, instead it gets the size of the object referred to by this.name. Given in your code, this.name is null, the returned size is 0.

Given instances of Animal have 1 field (name), and instances of Dog have two fields (name, inherited from Animal, and dogName), instances of Dog will obviously need to be larger than instances of Animal.

Upvotes: 3

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