Class A() {
    Integer a1;
    Integer a2;
}

Class B() extends A {
    Integer b1;
}

Class A {
    protected Integer a1;
    protected Integer a2;
}

Class B extends A {
    Integer b1;
}

B b = new B();
b.a1 = 3;
b.a2 = 4;

A aReference = new A()
aReference.a1 = 1;
aReference.a2 = 2;
b1 is not present in the object so can not be set.

A aReference = new B(); //reference of parent class and object of child class.
aReference.a1 = 10;
aReference.a2 = 20;

((B)aReference).b1 = 30;

B bReference = new B();
bReference.a1 = 10;
bReference.a2 = 20;
bReference.b1 = 30;

Suppose you want to modify it from inside a method of B

 class B extends A{
     ....
    public void someMethod(){
       super.a1 = 10;
       super.a2 = 20;
       b1 = 40;
    }
 }

If you want to modify the values of the state of child from parent class

public class A{

     public void someMethod(){
         ((B)this).b1 = 10;
         a1 = 20;
         a2 = 30;
    }
}

class A {
    Integer a1=5;
    Integer a2=6;
}

class B extends A {

}

public class Inherit {

    public static void main(String[] args) {

        A obj = new B();     //casting
        A obj2=new A();

        obj.a1 = 10;
        obj.a2 = 12;


        System.out.println("value of a1 in class A :"+obj2.a1+" & value of a2 in class A :"+obj2.a2);
        System.out.println("value of a1 in class B :"+obj.a1+" & value of a2 in class B :"+obj.a2);
    }

}

B b = new B();
 b.b1 = 1;

super.a1 = 3; // must execute from B class