Type problem - Class Cast Exception - trying to understand

Question

hello,
i get a Class cast exception in the following code.
interface Foo {}
class Alpha implements Foo {}
class Beta extends Alpha {}
class Delta extends Beta {
public static void main( String[] args ) {
Beta x = new Beta();
Beta b = (Beta)(Alpha)x; //fine
Foo f = (Delta)x; //CCE
Object ob = new Object();
String s = new String();
ob = s; //fine, no need of case
s = (String)ob; //needs a castI was just thinking, implicitly Delta and Beta both of them IS-A Foo, and assigning x to a type foo shouldnt be a problem(while making it refer to the Delta class) so, what could cause the CCE ?
I would appreciate if someone could please help me understand this.
Rgds

Answer 1

Hi.
Assigning your "x" to a variable typed Foo is not a problem. The problem is that you are trying to cast an instance of Beta ("x") into type Delta which is not possible. Every Delta is a Beta, but not every Beta will be a Delta!
Simple as that.
Bye.