Polymorphism in Java error:cannot find Symbol

Question

I've just started learning object oriented programming from the book head first java.It said that polymorphism enables me to create an array of the superclass type and then have all the subclasses as the array elements.But when I tried writing code using the same principles it ran into error saying error: cannot find symbol I made the classes the superclass was animal and the dog class extended the animal class having a fetch method of its own, but when I referenced the dog variable as animal it did not work here is the code

The Animal class:

public class animal{
        
    String family;
    String name;
    
    public void eat() {
        System.out.println("Ghap Ghap");
    }

    public void roam() {
        System.out.println("paw paw");
    }

}

The dog class:

public class dog extends animal {
    public void fetch() {
        System.out.println("Auoooooooo");
    }
}

The Tester class:

public class tester {

    public static void main(String args[]){
        animal doggie = new dog();
        doggie.fetch();
        doggie.eat();
        doggie.roam();
    }

}

The error:

tester.java:4: error: cannot find symbol
doggie.fetch();
      ^
symbol:   method fetch()
location: variable doggie of type animal
1 error

Edit: Last time I asked this question I went home thinking the object doggie is of the type animal and it has no idea of about the fetch() function that has been declared in the dog class. But adding the line

System.out.println(doggie.getClass().getName()); 

Gives dog as the type of the class, if dog is indeed the type of the class, shouldn't it have the knowledge of the method declared within it ?

Answer 1

When using polymorphism, if you create an instance of the subclass and store its reference in a variable of superclass type, you can only call those methods on the newly created instance which are present in the super class.

In your code, you created an instance of dog class and stored its reference in doggie which is of type animal (super class of dog), In such case, you can't call any method on dog class instance that isn't available in animal class.

fetch method is not defined in the animal class hence you get the error.

Solution

Either define the fetch method in the animal class

OR

change

animal doggie = new dog();

to

dog doggie = new dog();