overriding question

[sweety sinha]

this question is from examlab

public class Fractal{

     public static void main(String args[]){
          Fractal ft=new SubFractal();
          String s=(Fractal)ft.getOb().getMsg();
          System.out.print(s);	
     }

     public Object getOb(){
          return new Fractal();
     }

     public String getMsg(){
          return "SuperStar";
     }

}


class SubFractal extends Fractal{

     public Fractal getOb(){
          return new Fractal();
     }

     public String getMsg(){
          return "SubStar";
     }

}

why it is giving compilation fail. please give me a detail explanation

thanks in advance

[Bob Wheeler]

It can't cast from String to Fractal:

 (Fractal)ft.getOb().getMsg();   

This works:

((Fractal)ft.getOb()).getMsg();

Nothing to do with overriding.

cheers
Bob

[Sagar Rohankar]

TellTheDetails

[yen hoang]

why don't you try to compile this code! The compiler will complain in line "String s=(Fractal)ft.getOb().getMsg(); " that :
_ Type mismatch: cannot convert from Fractal to String ( not in the same hierarchical tree )
_ The method getMsg() is undefined for the type Object
To make compiler happy, you must do st like this String s=((Fractal)ft.getOb()).getMsg();
You should comment at everywhere you can, to track the way it goes. Be sure about the overriding. Good luck!!!

[Fritz Guerilus]

yen hoang wrote:why don't you try to compile this code! The compiler will complain in line "String s=(Fractal)ft.getOb().getMsg(); " that :
_ Type mismatch: cannot convert from Fractal to String ( not in the same hierarchical tree )
_ The method getMsg() is undefined for the type Object
To make compiler happy, you must do st like this String s=((Fractal)ft.getOb()).getMsg();
You should comment at everywhere you can, to track the way it goes. Be sure about the overriding. Good luck!!!

This was a tricky question and I didn't understand the answer until now.
Thanks

[indra negi]

What I believe is the getOb() method of SubFractal is overriding the getOb() method of Fractal class (covariant return types), then why is it that invoking the getOb() method from ft object aways invokes the getOb() of Fractal and not SubFractal.

Please explain.

[Ankit Garg]

As bob explained (3 months ago) the cast will try to cast the result of the last method call (i.e. getMsg) into Fractal (but getMsg method's return type is String). That's why you are getting a compile time error. This is because the precedence of method call is more than type cast. That's why you need extra parenthesis to make it work (again explained by Bob)...

[indra negi]

Hi Ankit,

I got that thanks however I want to know one more thing that if I say the following statement which getOb() method will be called the superclass one or the subclass one.

Fractal f=ft.getOb();

[Ankit Garg]

The getObj method of SubFractal class will be called (as ft refers to an instance of SubFractal), but since the type of ft is Fractal, so at compile time the call will be matched to getObj in Fractal class (which returns Object) so you'll have to use a cast for compilation to succeed.

Fractal f=(Fractal)ft.getOb();

[Salil Vverma]

As we know that member access operator has higher precedence than the casting operator. The explanation of compiler’s behavior is as follows –

At the current state –
1- compiler sees that after possible execution of first member access operator ft.getOb() would return object (java.lang.Object) type. Compiler knows that object class does not have a function getMsg() so would give compilation error as “The method getMsg() is undefined for the type Object”
2- Compiler sees that this expression has member access operator and type cast operator. As the member access operator has higher precedence, type cast would be done at the end and then it would try to assign Fractal value to an String which would be incorrect so it would give another compilation error as “The method getMsg() is undefined for the type Object”

I hope this solves all the confusion of compilers behavior about this program.

Regards
Salil Verma

[indra negi]

Thanks Ankit. I got that.

[Faisal Fuad]

Hi...although this is a very old post...but can any one please tell me why i can't do the following in Fractal class:

String s=((SubFractal)ft.getOb()).getMsg()

It is showing the following ERROR if i do so:

Exception in thread "main" java.lang.ClassCastException: Fractal incompatible with SubFractal

Is this just because they are two different objects and their return types are not subtype of each other?
But getMsg() type is same for both right?

Currently it is printing "SuperStar"....what i have to do if would like to print "SubStar"?


Thanks

[Faisal Fuad]

Got the solution....it was my bad. I was confusing myself with Object Type and Reference Type and how the Compiler and JVM do behave accordingly with each one of them
Below code changes will print "SubStar" String as final result.

public class Fractal{   
     public static void main(String args[]){   
          SubFractal ft=new SubFractal();   
          String s=((Fractal)ft.getOb()).getMsg();   
          System.out.print(s);     
     }   
  
     public Object getOb(){   
          return new Fractal();   
     }   
  
     public String getMsg(){   
          return "SuperStar";   
     }   
}   
  
class SubFractal extends Fractal{   
  
     public SubFractal getOb(){   
          return new SubFractal();   
     }   
  
     public String getMsg(){   
          return "SubStar";   
     }   
}

Many many thanks for all the critical explanations related to this topic.