Problem with ClassCastException

[Sudhanshu Gupta]

hi all,
I was taking the self test in SCJP study guide where this code confused me.

class Tree {

}

public class Redwood extends Tree
{
    public static void main(String []arg)
    {
        new Redwood().go();
    }
     void go()
    {
        go2(new Tree(),new Redwood());
        go2((Redwood)new Tree(),new Redwood());
    }

     void go2(Tree t1, Redwood r1)
    {
    Redwood r2 = (Redwood)t1;
    Tree t2 = (Tree)r1;
    }
}

This code throws ClassCastException when Tree is downcast to Redwood.
But I dont understand why. To try this out I used an existing code and tried upcasting and downcasting.
I dont understand how come this one is working fine.

class NewTest extends Test {
    
    public static void main(String []a)
    {
        Test t = (Test)new NewTest();
        t.doThis(5);

        Test t1 = new Test();

        NewTest n = (NewTest)t1;   //throws no exception
        n.x=9;

        t1 = (NewTest)new Test();
        n= (NewTest)t1;
    }
    int x;

   
}

Regards

Sudhanshu Gupta

[Ernest Friedman-Hill]

Sorry to say, my friend, but you are mistaken; your example is not working fine. It will compile, yes, but at runtime, the line

NewTest n = (NewTest)t1; //throws no exception

absolutely will throw a ClassCastException. A cast can never actually change an object; it can only tell the compiler something that is true. So the first cast, from a NewTest object to class Test, is OK, because a NewTest is-a Test. But the second cast fails, since the object is really an instance of Test, and can't be converted to NewTet. There's a third thing you can do which does work, and I think that's the interesting case that you're missing:

Test t2 = new NewTest();
NewTest t3 = (NewTest) t2;

You can't make the second assignment without the cast, but with the cast, it's legal, since t2 really does hold a NewTest instance.

[Sudhanshu Gupta]

hi Ernest Friedman-Hill
thank you very much for clearing my confusion.
Actually I was thinking of collections which return Object instances that can be cast to other types.
The third thing you told about clarified everything.
thank you once again.

Regards

Sudhanshu Gupta