How static method inheritance and static variable inheritance differs?

Question

I got a doubt while going through override functionality in java.

Consider the below code:

class Vehicle {
    static int speed = 50;

    public static void display() {
        System.out.println(speed);
    }
}

class Jeep extends Vehicle {
    int speed = 100;

    void display() {    //GETTING COMPILE TIME ERROR
        System.out.println(speed);//will print speed of Bike   
    }

    public static void main(String args[]) {
        Jeep b = new Jeep ();
        System.out.println(b.speed);

    }
}

I read that static methods cannot be overridden.

But in the above code, I declared a static variable 'speed' in parent class Vehicle. And I created an instance variable with same name 'speed' in child class. i didn't get any compile time error as I changed the value of the static variable 'speed' in child class.

I am facing compile time issue while trying to override the display method, while I am not getting any error while re-declaring the variable 'speed' even though both are static in parent class.

What may be the reason that, the subclass's speed variable hides the parent class's static speed variable , but not doing the same with the display method and showing compile time error ?

Which are you asking about? Instance fields or methods? `speed` or `display`? (It matters.) — T.J. Crowder, Sep 29 '16 at 13:53
you didn´t change anything of the static variable `speed` in the child class? — SomeJavaGuy, Sep 29 '16 at 13:53
Fields can never be overridden, whether static or not, so there is no problem for fields. — Erwin Bolwidt, Sep 29 '16 at 13:53
http://stackoverflow.com/questions/10501503/java-field-hiding — brso05, Sep 29 '16 at 13:54
Possible duplicate of [Why doesn't Java allow overriding of static methods?](http://stackoverflow.com/questions/2223386/why-doesnt-java-allow-overriding-of-static-methods) — Ravindra babu, Sep 30 '16 at 14:56

score 3 · Answer 1 · answered Sep 29 '16 at 13:53

3

static functions are not part of a particular instance of an object, and polymorphism doesn't really make much sense unless it's applied to an object.

That's why you cannot override a static function.

answered Sep 29 '16 at 13:53

Bathsheba

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score 1 · Answer 2 · edited May 23 '17 at 12:03

In your code the subclass defines a field with the same name(speed) which is declared as a new field and the field in the superclass is hidden or in other words shadowed by subclass field with same name.

A shadowed field indicates that it is not inherited by the subclass instead the subclass has declared a field with same name in its scope.

Hiding of field does not affect its value in the superclass. To access the hidden field simply use super.fieldname (Vehicle.speed in your case since its static).

Fields cannot be overidden but only be hidden in java.

Shadowing of fields is considered as bad practice by many as it makes the code less readable and confusing.

As for why static fields cannot be overridden Bathsheba has provided an excellent answer on your question post or refer Why doesn't Java allow overriding of static methods?

How static method inheritance and static variable inheritance differs?

2 Answers2