which method to use?

[Paul Villangca]

from sanjayahuja.tripod.com (with modifications)

public class AQuestion 
{ 
     public void method(Object o) 
     { 
         System.out.println("Object Verion"); 
     } 
// public void method(String s) 
// { 
//    System.out.println("String Version"); 
// }
     public void method(Integer i)
     {
System.out.println("Integer Version");
     }
     public void method(Number i)
     {
System.out.println("Number Version");
     }
 public static void main(String args[]) 
 { 
     AQuestion question = new AQuestion(); 
     question.method(null); 
 }
} 

I thought this would be a compile error, 'coz method(null) would be ambiguous (it wouldn't know which method() will be invoked.) But it compiles and prints out "Integer Version" when run! Also, when method(String) is uncommented out, then the compiler complains about ambiguous methods.

[Tybon Wu]

Note the inheritance tree:
Object
|
+-Number
++|
+++-Integer
Since null is compatible to any object, the one furthest down the compatible path will be chosen, in this case, Integer.
If you also have the overloaded method with the String parameter, then that will create two compatible paths for null, which causes the ambiguous methods error.
[ June 19, 2002: Message edited by: Tybon Wu ]