New to Java

As I was playing around with some code I came across this point :

First Class

public class A {
	public int x;
	A(int i){
		x=i;
		System.out.println("A is initialised");
	}

}

Second Class extending it :

public class B extends A{
	private int y;
   // Why do I need this constructor to call parents constructor?
  // My guess is so that when i make an object of class B referring to class A it should make sense?
	B(int i) {                      
		super(i);  
		y=test;
	}

	
public static void main(String args[]){
	 A a = new A(1);
	 A b = new B(1); make an object of class B referring to class A it should work!!
	 B c = new B(2);
	 B d =(B) new A(2);  --> gives class cast exception!
}

I am little confused here, Can someone throw more light on it.

Thanks