Very hard question for you Java Experts

[Anonymous]

From the Mughal test:
Which statements , when inserted at the indicated
postion, will cause a runtime exception when attempting
to run the program.
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 here
}
}
////////////////////////////////////////////
Select All valid Answers:
(A) x= y;
(B) z=x;
(C) y=(B) x;
(D) z=(C) y;
(E) y=(A) y;
( I do not understand why ( the right answer or answers ) is
correct )

[Frank Wang]

I think the answer is (C) y=(B) x. Only A and C can be compiled. But C gets a runtime exception when runs, because x is an instance of A, not B.

[Anonymous]

Frank is right,the answer should ne a. bcoz u are it will be implicitly cast towards the upside of hierachy, as we are assigning object y to x.Correct me if i am wrong.

--Shallender

[Minoru Horie]

Originally posted by Shallender:
Frank is right,the answer should ne a. bcoz u are it will be implicitly cast towards the upside of hierachy, as we are assigning object y to x.Correct me if i am wrong.

--Shallender

do u mean the answer should be c? I think it is c

[sanjay gautam]

hello every body,
there is a lot of doubt regarding object casting.in the question
b),d) and e) will give compilation error.
always remember derived class object can be assigned to base class object but vice versa is is not correct . for this casting is required.
class A{}
class B extends A {}
class C extends A {}
public class Test {
public static void main(String[]s) {

A x =new A();
B y =new B();
C z =new C();

x = y; //no error
x = z; //no error
y = z; //error becoz there is no relation between class B and class c.

y =(B)x; //no error
z =(C)x; //no error
y =(B)z; // error becoz there is no relation between class B and class c. even casting cannot help.
z =(C)y; // error becoz there is no relation between class B and class c. even casting cannot help.
y =(A) y; //error
z =(A) z ;//error

}}
hoping things will be more clear now if i am wrong please write me. waiting for response.

gautam

[Anonymous]

I agree with Sanjay and Frank, but I want to add that even though you can get a class to compile by adding a cast, you will still get the RuntimeException when trying to cast from a base class to a derived class. Even with casts, you cannot move "down" the heirarchy tree (right?). Casting just gives you more flexibility when compiling. Generally speaking, if an operation would fail conversion rules, it will fail to cast at runtime as well. For instance this will compile, but gives ClassCastException at runtime:
Object o = new Object();
String s; //Derived from object
s = (String) o;

[Anonymous]

No the answer is C.

[Anonymous]

thanks all!
the answer is c

[Marcela Blei]

Sorry for saying this but, this is not a question for experts, is a very basic one, you have to study this topic in deep before taking the exam, it�s very important.

[hanmeng]

Hi, I have try to compile all the situation. the Result is show following. but I don't know why the (c) compile good, runtime is error.
==============================
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();
// x = y; //compile and run good
// z = x; //compile error: Q3ae4.java: 18:
//incopatible types
//found : A
//required: C
//z = x;
/* y = (B)x;//compile good, runtime error
exception in thread "main" java.lang.ClassCastException: A
*/
/* z = (C)y; //inconvertible types
// found : B
// required : C

*/

/* y = (A)y; // incompatible types
// found : A
// required: B
*/
}
}