Method invocation with null argument

[Alexan Kahkejian]

Hi all
This code is small part of Dan's Questions:

  
public class A{
    static void m(Object o){System.out.print("Object");}
    static void m(String s){System.out.print("String");}
public static void main( String[] s ){
m(null);
}
}

This code prints "String". Why?
So how we can know that which overloaded method will be invoked when the argument is null.
Thanx in advance
Alexan
[ July 01, 2003: Message edited by: Alexan Kahkejian ]

[Valentin Crettaz]

The most specific method is invoked. In this case, the method taking a String argument is more specific than a method taking an Object argument.
Please see the following posts which deal with the same topic:
http://www.coderanch.com/t/239328/java-programmer-SCJP/certification/null-argument-method-being-passed
http://www.coderanch.com/t/239534/java-programmer-SCJP/certification/null-arg-overloaded-method
Also check out JLS 15.12.2.2 Choose the Most Specific Method to find out about how methods are resolved.

[Aravind Elango]

Hi,
The String class is a subclass of the object. When the method m is called with null, the subclass gets called. I guess this could be a general rule that a subclass gets called before superclass.

[Brian Joseph]

What I do, and I hope this is basically the right approach, is look at type of the argument/s and ask myself which one can be implictly cast to all the others. If there is a tie between two or more, then it is a compile error (ambiguous).