overload error when inherit in java?

Question

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Animal a=new Animal();
        a.speak();

        Animal d=new Dog();
        d.speak(3);//           
    }

    class Animal
    {
        public float speak()
        {
            System.out.println("I am a animal!");
            return 0;
        }
    }

    class Dog extends Animal
    {
        public double speak(int a)
        {
            System.out.println("Dog sparks!");
            return 0;
        }
    }
}

I just learned override and overload in java. Here what i am trying to do is to implement a overload.But the compiler shows that d.speak(3) is wrong.

I think the dog inherit the animal. So it has a speak() func. And I add a speak(int) to overload. Why I am wrong? thank u.

Reimeus · Accepted Answer · 2014-09-25 18:27:57Z

4

Only Dog can speak with an int argument.

Dog dog = new Dog();
dog.speak(3);

If you want to call speak as an Animal without a cast, then the argument list needs to be the same for all objects

edited Sep 25, 2014 at 18:27

answered Sep 25, 2014 at 18:08

Reimeus

160k16 gold badges225 silver badges282 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Nicola Miotto · Accepted Answer · 2014-09-25 18:10:42Z

2

It's because the Animal class doesn't have a speak method with that signature (taking an int parameter). Therefore the compiler cannot find the definition in the Animal class. You have to downcast it before, like

Animal d = new Dog();
((Dog)d).speak(3);

EDIT: or, as Reimeus suggested, declare it directly as a Dog.

answered Sep 25, 2014 at 18:10

Nicola Miotto

3,7262 gold badges31 silver badges43 bronze badges

2 Comments

AlexWei Over a year ago

So when I want to implement polymorphism,first I have to ensure the inherit tree has same signature. Or I have to 'cast' it to proper data types?

Nicola Miotto Over a year ago

@AlexWei Polymorphism applies just with the same signature. So if the method interface (given by name, return type and parameters names and types) is the same throughout the classes in the hierarchy, you can call that method from a pointer to the base class (in this case Animal) and the runtime will invoke the method on the actual class (the Dog in this case) of the instance.

Jai Bhatt · Accepted Answer · 2014-09-25 18:21:43Z

0

You are using reference of Animal and instance of Dog.
In case of overridden methods, it points to the method available in referenced class.
Here, it will point to speak() method of Animal class because it is the referenced class.
Therefore, it is unable to find parametrized method speak(int a).

//Points to speak() of Animal
Animal a=new Animal();
a.speak();

//Points to speak(int ) of Dog class
Dog d=new Dog();
d.speak(3);

//Points to speak() of Animal class
Animal d=new Dog();
d.speak(3); //Results in compilation error as speak(int) is not available with Animal reference

This situation is called SuperClass reference for SubClass object in object oriented programming terminology.

answered Sep 25, 2014 at 18:21

Jai Bhatt

911 silver badge2 bronze badges

Comments

evanwong · Accepted Answer · 2014-09-25 18:23:05Z

0

You are overloading the speak method in Dog. And Java use static binding for overloading method which means the declared type has to have the overloaded method. In your case, Animal is the declared type but it doesn't have the speak(int a). And that why you have to type cast it: ((Dog)d).speak(3) to make the compiler stop complaining.

answered Sep 25, 2014 at 18:23

evanwong

5,1543 gold badges34 silver badges45 bronze badges

1 Comment

AlexWei Over a year ago

I just searched static binding in your answer. It really helps.

Collectives™ on Stack Overflow

overload error when inherit in java?

4 Answers 4

Comments

2 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

2 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related