casting doubt

[pritam kamkar]

hey people i got this question on the mock exam

class X { void go() { System.out.println("x"); } }
class Y extends X { void go() { System.out.println("y"); } }
class Z extends X { void go() { System.out.println("z"); } }
class Chrome2 {
public static void main(String [] args) {
X z = new z();
Z y = new Y();
Z y2 = (Z)y;    #line1
z.go();
y.go();
y2.go();
}
}

the answer says that the cast on line1 will fail at runtime can anyone please tell me why does it work during compile time

[Sunny Mattas]

[B]   code:
 
    class X { void go() { System.out.println("x"); } }
    class Y extends X { void go() { System.out.println("y"); } }
    class Z extends X { void go() { System.out.println("z"); } }
    class Chrome2 {
    public static void main(String [] args) {
    X z = new z();
    Z y = new Y();
    Z y2 = (Z)y;    #line1
    z.go();
    y.go();
    y2.go();
    }
    }
 

 
the answer says that the cast on line1 will fail at runtime can anyone please tell me why does it work during compile time[/B]

At compile type, compiler doesnot know about the real object at line1,thing it only knows about is that y is a referance of class z. so casting z into z will have no problem.

Now at run time real object is of class y and there is no inheritance between z and y.Ttherefore it will give an error.

Regards
Sunny

[Ashish Hareet]

Pritam,

Z y = new Y();
will not compile, Z & Y don't belong in the same class hierarchy.

Where is this question from ?

[Raphael Rabadan]

Originally posted by pritam kamkar:
hey people i got this question on the mock exam

class X { void go() { System.out.println("x"); } }
class Y extends X { void go() { System.out.println("y"); } }
class Z extends X { void go() { System.out.println("z"); } }
class Chrome2 {
public static void main(String [] args) {
X z = new z();
Z y = new Y();
Z y2 = (Z)y;    #line1
z.go();
y.go();
y2.go();
}
}

the answer says that the cast on line1 will fail at runtime can anyone please tell me why does it work during compile time

Hello, this is a question from Bonus Master Exam i guess, and you might have copied wrong, because the line that gives an error in your exmaple is the line above line1.

I think the original is:

class X {
void go() {
System.out.println("x");
}
}
class Y extends X {
void go() {
System.out.println("y");
}
}
class Z extends X {
void go() {
System.out.println("z");
}
}
class Chrome2 {
public static void main(String[] args) {
X z = new Z();
X y = new Y();
Z y2 = (Z) y; #line1
z.go();
y.go();
y2.go();
}
}

and it fails on line 1 because y is of the type Y and it fails on the is a relationship, Y can't be casted to a type Z, it will fail in the test y instanceof Z.

I hope it helps.

Kind Regards,
Raphael Rabadan

[Bob Ruth]

I think a simple way to think of it is like this: a cast will fool a compiler into putting anything into a reference without squawking about it. But, at run time, the JVM knows what kind of object it REALLY is and will give you the old ClassCastException when it isn't a match.

[Sazzad Hossain]

To Raphael Rabadan
Does it mean casting is not possible if there is no is-a relationship in two classes?

[Raphael Rabadan]

Originally posted by Sazzad Hossain:
To Raphael Rabadan
Does it mean casting is not possible if there is no is-a relationship in two classes?

Yes. You can't use instanceof too.
See this code...

class A extends C {}
class B extends C {}
class C {}
 
public class TestingCast2 {
 
public static void main(String... args) {
A aAsA = new A();
B bAsB = new B();
C bAsC = new B();
A newA = (A) bAsB;
A anotherA = (A) bAsC;
}
}

1st Case A newA = (A) bAsB; - Here we can't cast because the reference type of bAsB is B and newA is A and A won't pass in the relationship test A is an B

2nd Case A anotherA = (A) bAsC; - Here we can cast because the reference type of bAsC is C and anotherA isA A and A pass in the relationship test A is an C. You'll get ClassCastException on runtime here. Because, at runtime the compile will see that C was an B, and A isn't an B, so, can't be casted.



Try this other code too..

class X {
void go() {
System.out.println("x");
}
}
class Y extends X {
void go() {
System.out.println("y");
}
}
class Z extends X {
void go() {
System.out.println("z");
}
}
public class TestingCast {
 
public static void main(String[] args) {
 
Z z = null;
Y y = null;
 
Z = y;
Y = z;
 
y = (Y) z;
z = (Z) y;
 
boolean a = (z instanceof Y);
boolean b = (y instanceof Z);
}
}

Kind Regards,
Raphael Rabadan
[ July 24, 2008: Message edited by: Raphael Rabadan ]

[Sazzad Hossain]

Make sense. Thanks a lot for clarification