• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Jeanne Boyarsky
  • Ron McLeod
  • Paul Clapham
  • Liutauras Vilda
Sheriffs:
  • paul wheaton
  • Rob Spoor
  • Devaka Cooray
Saloon Keepers:
  • Stephan van Hulst
  • Tim Holloway
  • Carey Brown
  • Frits Walraven
  • Tim Moores
Bartenders:
  • Mikalai Zaikin

Unable to run the following program

 
Greenhorn
Posts: 3
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
class Super
{
Super()
{
System.out.println("Constructor for Super class");
}

}

public class Sub extends Super
{
Sub()
{
System.out.println("Constructor for Sub class");
}
public static void main(String[] args)
{
Sub subObj=(Sub)new Super();
}
}

Hi,
The above program(Sub.java) is compiling but while running it is giving ClassCastException.. Why???
 
Ranch Hand
Posts: 2023
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
An instance of Super class is not an instance of its subclass Sub. You can not cast it to Sub.
[ August 25, 2006: Message edited by: wise owen ]
 
Ranch Hand
Posts: 115
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
As far as compilation is concerned, compiler doesn't know anything about
'new Super()' only JVM can know which object is being created. Compiler doesn't have any problem because you are downcasting 'something' to 'Sub' and assigning the value to 'Sub' reference which is perfectly acceptable for compiler but when it comes to JVM (at runtime) it complains that

"I can't downcast Super "object" (not reference) to another class (even to its subclass)."

downcasting in object reference is acceptable in following case:

//code
Super superReference= new Sub();
Sub subReference=(Sub) superReference;

because here we are downcasting super class reference (which refer to sub class object) to sub class one.
 
Enthuware Software Support
Posts: 4810
52
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Good explanation, Vaibhav.
When you put an explicit cast Sub subObj=(Sub)new Super();, you are telling the compiler to ignore the type of the reference and that you are make the compiler believe that at runtime the reference will point to the correct object.

However, in your case, at runtime the reference is not pointing to an object of class Sub (or its subclass). So the JVM throws the exception.

Note: You can think of this statement: Sub subObj=(Sub)new Super();
as :
Super tempReference = new Super();
Sub subObj=(Sub)tempReference;
 
Vaibhav Chauhan
Ranch Hand
Posts: 115
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
thanks paul.
 
With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.
reply
    Bookmark Topic Watch Topic
  • New Topic