What is the type on null as a method argument?

Question

So here it is this example

public static void main(String[] args) {        
    new Stuff(null);
    new Stuff("a");
    new Stuff(1);           
}

and class Stuff is defined as follow

public class Stuff {    
       Stuff(Object o){
           System.out.println("object");        
       }

       Stuff(String s){
           System.out.println("string");        
       }

}

The output is

string
string
object

How does Java tell the null is a String? If I change Stuff to

public class Stuff {    

       Stuff(String s){
           System.out.println("string");        
       }

       Stuff(Integer o){
           System.out.println("Integer");           
       }

}

I get compilation error for Stuff(null):

The constructore Stuff(String) is ambigous. 

Again, why does Java "decide" null is a String?

Answer 1

• The compiler first lists all applicable methods. In your case, both are applicable.
• It then tries to find a method which is more specific than the other(s).
• In your first example, String is a subclass of Object and is therefore more specific.
• In your second example, both methods are applicable (String and Integer) but neither is more specific than the other (String is not a subclass of Integer which is not a subclass of String). So there is an ambiguity, hence the compiler error.

The full algorithm to determine which method should be chosen is defined in the JLS.