CODEWITHSUNDEEP
java
sunny
sunny

Reputation: 223

java member variables' visibility between parent class and child class

I created the following code:

class Father {  
    int num = 10;  

    public void setNum(int num) {  
        this.num = num;  
    }  
    public int getNum() {  
        return num;  
    }  
}  
class Child extends Father {  

    public void show() {  
        System.out.println(num);            // 20
        System.out.println(this.num);       // 20   
        System.out.println(super.num);      // 20 
    }  
}  
public class Test001 {  
    public static void main(String[] args) {  
        Child child = new Child();  
        child.setNum(20);  
        child.show();  
    }
}

My question is why super.num will print 20? I think super.num is 10. Because variable num is unrelated between Father and Child. The variable num in Child is just a copy from Father. who can tell me why?

Upvotes: 1

Views: 1138

Answers (5)

daiscog
daiscog

Reputation: 12077

The variable num is not a copy of the variable in the parent class. It is inherited from it.

Note that num is an instance variable. One copy of it exists per instance of Father. Because Child extends Father, every instance of Child is an instance of Father, but with the added functionality defined by the Child class.

Upvotes: 0

prabodhprakash
prabodhprakash

Reputation: 3927

Child class inherits num from Parent class. It does not have a copy, but is truly the owner of this class.

I will give you example - Lets say parent is Fruit and num is taste - when a clild (say apple) inherits fruit - it owns the property taste. Even if "taste" was defaulted to "neutral" - apple will change taste to "appleTaste" and whenever you eat (i.e. print apple.taste) even using super, it will output "appleTaste" and not the default taste value. It is important to understand that "num" or "taste" belong to "instance" and thus, the final instance "apple" or "child" decides the value.

Gave you this example from real world to understand it via analogy and something that you can imagine.

Upvotes: 4

Sakalya
Sakalya

Reputation: 578

super.num will print 20 because those keyword "super" indicates the refernce of Parent Class,it is instantiating the sublclass. e.g.It is equivalent of ParentClass p = new ChildClass();

The refernce(p) is of Parent Class but it is still pointing to Subclass object.

Upvotes: 0

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62874

Because variable num is unrelated between Father and Child

That is not true. Child extends the definition of Father, which includes the member num. It's accessible from child-classes, so they are allowed to change it's value (directly), or by calling the setter method (which is also accessible from sub-classes of Father)

Upvotes: 1

user6420285
user6420285

Reputation:

The variable num in Child is just a copy from Father.

No it absolutely is not. num is defined in the parent class Father. It's accessible by Child due to the inheritance.

When you write child.setNum(20); it's actually the function in Father that is used.

Upvotes: 2

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