ClassCastException on Down Casting

[Santosh Kumar]

Hello friends
I have a very basic question.Just see the below code.

class Super
{
  public Super()
  {
     System.out.println("default Super");
  }
  public Super(String a)
  {
     System.out.println("Super -->"+a);
  }
 
  public void abcSuper()
  {
     System.out.println("Super method call ");
  }
}
 
class Sub extends Super
{
  public Sub()
  {
    System.out.println("Default Sub");
  }
  public Sub(String a)
  {
    System.out.println("Sub -->"+a);
  }
 
  public void abcSub()
  {
    System.out.println("Sub method call ");
  }
 
  public void abcSuper()
  {
    System.out.println("Super Sub method call ");
  }
}
 
public class Inherit
{
  public static void main(String[] arg)
  {
    Inherit in = new Inherit();
 
    Super supe = new Super();
    Sub sub = new Sub();
 
    sub = (Sub) supe ;
    sub.abcSuper();
  }
}

Now tell me one thing when i tried to compile this program it was compiled successfully.But at run time it had shown on console

default Super
default Super
Default Sub
Exception in thread "main" java.lang.ClassCastException
        at Inherit.main(Inherit.java:54)

Now tell me in java its perfectly allowed to downcast any class from super class,then why at runtime it is throwing this ClassCastException
waiting for reply.
thank you
[ edited to help preserve code formatting us the [code] and [/code] UBB Tags -ds ]
[ June 05, 2003: Message edited by: Dirk Schreckmann ]

[Bear Bibeault]

    Super supe = new Super();
    Sub sub = new Sub();
    sub = (Sub) supe ; 

You can cast a Sub to a Super. Since Sub extends Super, any instance of a Sub is a Super (but with extra stuff only available to a Sub).
However, supe is a Super. Since a Super is not a Sub, you can't cast an instance of a Super to a Sub.
So is your question about why this threw an exception (which it should since supe is not a Sub)? Or why the compiler doesn't complain?
hth,
bear
[ June 05, 2003: Message edited by: Bear Bibeault ]

[Santosh Kumar]

thank you for reply.
But still now one question i have.As Object is super class of all the java classes.So we cant typecast Object with these classes as it will throw ClassCastException.
Now tell me one thing like I have one method which is very generic, takes Object as Parameter and returns object of type Object.So if I try to typecast this returned Object with my own object then it will throw exception.
So now you tell me how i am going to solve this problem and also also method should be generic so that i dont have to write different method with same code for different object return types.

Thank you
Regards

[Richard Jensen]

Originally posted by santosh kumar:
But still now one question i have.As Object is super class of all the java classes.So we cant typecast Object with these classes as it will throw ClassCastException.
Now tell me one thing like I have one method which is very generic, takes Object as Parameter and returns object of type Object.So if I try to typecast this returned Object with my own object then it will throw exception.
So now you tell me how i am going to solve this problem and also also method should be generic so that i dont have to write different method with same code for different object return types.

I'm not sure I quite understand your problem. Given classes that are related by inheritance, you can safely downcast only when the object actually pointed to at the time of the downcast is of the specified type. Otherwise you get the ClassCastException.
So:

class Super extends Object {
}
class Derived1 extends Super {
}
class Derived2 extends Super {
}
public static void main(String args[]) {
   Super supObj = new Super();
   Derived1 d1Obj = new Derived1();
   Derived2 d2Obj = new Derived2();
   Super anObj = supObj;
   Derived1 d1Temp = null;
   d1Temp = (Derived1) anObj; // NO!
   // At this point anObj cannot be downcast because it is actually a Super
   anObj = d1Obj; // upcasting is safe [no cast required
   d1Temp = (Derived1) anObj; // YES!
   // At this point anObj is actually a Derived1
   anObj = d2Obj;
   diTemp = (Derived1) anObj; // NO!
   // At this point anObj is actually a Derived2
}

Does that help at all?

[Dirk Schreckmann]

santosh,
You might enjoy the "How my Dog learned Polymorphism" story in The JavaRanch Campfire Stories. It doesn't deal explicitly with casting, but it might help get ya started with a few of these concepts.

[Santosh Kumar]

There is a term downcasting in java then when it is allowed.I know it is allowed in primitive data types but where else it is not.
Can you tell me ?
Thank you

[Thomas Paul]

You can downcast only when an object is actually the object you are downcasting to. For example:
Object a = new Button();
Button b = (Button)a;
This is a perfectly valid downcast because "a" really is a Button.
So if a method returns an Object but you know that Object is really a PotatoChip then downcasting to a PotatoChip is valid and will work.
For example:
PotatoChip pc = (PotatoChip)myArrayList.get(0);
As long as the ArrayList really contains a PotatoChip this will work.