Method Overriding...

[Pankaj Patel]

Problem : The Constructor sample(String) is ambigious. How?

package SCJP;
 
class sample
{
sample(String s)
{
System.out.println("String");
}

sample(StringBuffer sb)
{
System.out.println("StringBuffer");
}
}
 
public class Constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}

[Vassili Vladimir]

The compiler will complain cause either constructors can be passed a null, it will not be able to distinguish the right constructor to invoke!

Best of luck ...

[Keith Lynn]

sample(String) isn't ambiguous.

sample(null) is ambiguous.

null is accepted by both constructors, but neither constructor is more specific than the other.

[Pankaj Patel]

Hi Keith, So according to you the following code should also give error. But it does not give any error. Why?

package SCJP;
 
class sample
{
sample(String s)
{
System.out.println("String");
}

sample(Object o)
{
System.out.println("Object");
}
}
 
public class Constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}

Thanks for efforts.

[Tina Tibrewal]

This wont give any error because of the inheritance relation between
String and Object.
It will call the more specific class that is the String Constructor.
[ April 20, 2007: Message edited by: Tina Tibrewal ]

[Pankaj Patel]

So, in the case of String and StringBuilder, both are at the same level of hierarchy, i mean both are extended from Object. That is why it gives error. Accepted.

But what in the case of Integer and String argument type.

: String -> Object
: Integer -> Number -> Object

// -> Indicates Extends From

package SCJP;
 
class sample
{
sample(String s)
{
System.out.println("String");
}

sample(Integer s)
{
System.out.println("Integer");
}
}
 
public class Constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}

But Still Compiler error Resides. Can you explain me in this case?
Thanks in Advance.
[ April 20, 2007: Message edited by: Pankaj Patel ]

[Chandra Bhatt]

package SCJP;

class sample
{
sample(String s)
{
System.out.println("String");
}

sample(Object o)
{
System.out.println("Object");
}
}

public class Constructor
{
public static void main(String arg[])
{
sample s1=new sample(null);
}
}

In case of sample(Object o) and sample(String o):

String IS-A(n) Object:
so calling a constructor with String arg is more specific that Object; because what is passed to an String can also be passed to object and vice versa is not true; so no ambiguity and call to sample(String args) is made.

In case of sample(String s) and sample(StringBuffer s):

There is no relationship between String and StringBuffer:
null can be substituted to the String and StringBuffer too, so the compiler is confused what to do, which constructor to call. And finally compiler complains that this is ambiguous call;




Regards,
cmbhatt

[Cory Max]

StringBuilder is a subclass of Object
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/StringBuilder.html

String is also a subclass of Object:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/String.html

This means they are unrelated. When you pass in null, the complier does not know which one to call.

If there are two objects that are related (one is a subclass of the other), the most specific one will be called. This is the case with Object and String.

Hope this helps.

Cory

[Pankaj Patel]

Thanks you very much Everybody. I finally Got it.