why can not access child fields using parent reference

Question

class A {
    int super_var = 1;
}

class B extends A {
    int sub_var = 2;
}

public class Demo{
    public static void main(String []args){        
        A a = new          


        

Answer 1

Let's say you have these classes:

public class Animal() {
    // ...
}

public class Fish extends Animal() {
    public void swim() {...}
}

If you declared an Animal:

Animal x = new Fish();

and you called the swim() method

x.swim();

Would you expect it to work? I don't think so, because not every animal can swim. That's why you have to explicitly specify that the animal x is a Fish:

((Fish) x).swim();

In your case, if you wanted to call that method, you should specify (technically, it's called cast) the type:

System.out.print(((B)a).sub_var);

Note:

• This works similar for methods and variables. I used a method in the example since it's more illustrative.

Edit:

Let's see this example:

Animal x;

if (some_condition)
    x = new Fish();
else
    x = new Cat();

x.swim();

This restriction exists, because Java won't know if, at execution time, the object assigned to x will have the method swim(). So to avoid this, you have to cast to the respective type to call a method that doesn't exist in superclass.