ClassCastException Doubts

[Micheal John]

From web page

Which statements, when inserted at the indicated position in the following code, will cause a runtime exception?
 
class A {}
class B extends A {}
class C extends A {}
public class Q3ae4 {
    public static void main(String[] args) {
        A x = new A();
        B y = new B();
        C z = new C();
        // insert statement here
    }
}

1. x = y
2. z =x;
3. y = (B)x;
4. z = (C)y;
5. y = (A)y

In the above I know 3rd will cause runtime exception.
Why y = (A)y is compile time exception. pls explain..

[Bob CHOI]

y=(A)y;

y on the left has type "B",
(A)y on the right has type "A",
"A" isn't a "B", the assignment fails, so compiler detects type mismatch

[Sanjeev Singh]

1. x = y
This is OK as a super class reference can hold the subclass reference(can refer to the object which subclass was refering) without any explicit cast.
So will pass both the compile time and runtime test.
2. z =x;
require explicit cast.Compilation fails.
3. y = (B)x;
x can be casted to y as both A and B are part of same inheritance lineup.Compilation succeds.Runtime fails downcasting not possible.
4. z = (C)y;
y can not be casted to z,not in the same inheritance hierarchy.
5. y = (A)y
Compilation fails sub class(B) can not hold the reference of super class (A) without a explicit cast. To make it work change something like y = (B)((A)y).