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

Is there a variable called sub_var in the parent class ? No. That is why you get the error -

sub_var cannot be resolved or is not a field

See this

System.out.print(a.super_var); //okay
System.out.print(a.sub_var); //compile error