What is the difference between up-casting and down-casting with respect to class variable

Question

What is the difference between up-casting and down-casting with respect to class variable?

For example in the following program class Animal contains only one method

Answer 1

1.- Upcasting.

Doing an upcasting, you define a tag of some type, that points to an object of a subtype (Type and subtype may be called class and subclass, if you feel more comfortable...).

Animal animalCat = new Cat();

What means that such tag, animalCat, will have the functionality (the methods) of type Animal only, because we've declared it as type Animal, not as type Cat.

We are allowed to do that in a "natural/implicit/automatic" way, at compile-time or at a run-time, mainly because Cat inherits some of its functionality from Animal; for example, move(). (At least, cat is an animal, isn't it?)

2.- Downcasting.

But, what would happen if we need to get the functionality of Cat, from our type Animal tag?.

As we have created the animalCat tag pointing to a Cat object, we need a way to call the Cat object methods, from our animalCat tag in a some smart pretty way.

Such procedure is what we call Downcasting, and we can do it only at the run-time.

Time for some code:

public class Animal {
    public String move() {
        return "Going to somewhere";
    }
}

public class Cat extends Animal{
    public String makeNoise() {
        return "Meow!";
    }   
}

public class Test {

    public static void main(String[] args) {
        
    //1.- Upcasting 
    //  __Type_____tag________object
        Animal animalCat = new Cat();
    //Some animal movement
        System.out.println(animalCat.move());
        //prints "Going to somewhere"
        
    //2.- Downcasting   
    //Now you wanna make some Animal noise.
        //First of all: type Animal hasn't any makeNoise() functionality.
        //But Cat can do it!. I wanna be an Animal Cat now!!
        
        //___________________Downcast__tag_____ Cat's method
        String animalNoise = ( (Cat) animalCat ).makeNoise();
        
        System.out.println(animalNoise);
        //Prints "Meow!", as cats usually done.
        
    //3.- An Animal may be a Cat, but a Dog or a Rhinoceros too.
        //All of them have their own noises and own functionalities.
        //Uncomment below and read the error in the console:
        
    //  __Type_____tag________object
        //Cat catAnimal = new Animal();
        
    }

}