what is "null"

[Angell Bulin]

why the program1 have compile error, but program2 can compiled?
i think "null" is a object, is wrong?

//------program 1
public class Tester {
void test(Object s) { System.out.println("Object"); }
void test(SubTester s) { System.out.println ("SubTester"); }
void test(String s) { System.out.println ("String version"); }
public static void main (String args[]) {
Tester c = new Tester ();
}
}
class SubTester extends Tester{ }

//------program 2
public class Tester {
void test(Object s) { System.out.println("Object"); }
void test(Tester s) { System.out.println ("Tester"); }
void test(SubTester s) { System.out.println ("SubTester"); }
public static void main (String args[]) {
Tester c = new Tester ();
}
}
class SubTester extends Tester{ }

[Valentin Crettaz]

I see nothing wrong with both of your code... I tried them and they compiled and ran fine... without error...
Besides, null is not an object, null just means that a reference variable does not reference any object...

[chafule razgul]

interestingly System.out.println(null); is not as innocent as it seems

[Marilyn de Queiroz]

I have no compile error with either program, but also no output. Here is a version with output.

public class Tester1 
{
    void test( Object o ) 
    { 
        System.out.println("Object"); 
    }
    void test( Tester1 t ) 
    { 
        System.out.println("Tester"); 
    }
    void test( SubTester st ) 
    { 
        System.out.println("SubTester"); 
    }
    void test( String s ) 
    { 
        System.out.println("String version"); 
    }
    
    public static void main ( String args[] ) 
    {
        Tester1 tester = new Tester1();
        tester.test( new Object() );
        tester.test( tester );
        tester.test( new SubTester() );
        tester.test( "" );
    }
}
    
class SubTester extends Tester1{ }

[ February 27, 2002: Message edited by: Marilyn deQueiroz ]

[Angell Bulin]

Valentin

if you compile and run these code, could you tell me the result :roll:

[Angell Bulin]

Marilyn
tester.test( new Object() );
tester.test( tester );
tester.test( new SubTester() );
tester.test( "" );
all these four call pass the explicit object as a parameter, but if you pass null to test(), which method will be used ?

[Marcus Howarth]

Very strange, I'd like an explanation too if poss.
adding the following call
tester.test( null );
to the above code gives a compiler error stating "Reference to test is ambigious .. " it's a String and a SubTester..
when I added the above line to the original code provided it happily compiled and when passing a null (as above) would execute the overloaded test(SubTester st) method at runtime.
But the addition of a method that takes a String has changed this. When a tester.test( null ); call is made now it doesn't compile.

what's happening here? is there some strange casting rule applied to null that implements the String and sub-class overloaded methods??
[ February 27, 2002: Message edited by: Marcus Howarth ]

[R K Singh]

Hope that helps
Regards
Ravish

[Marcus Howarth]

spot on!
Ambiguity comes from the fact that the compiler is looking for the most derived class. In this case it sees String and SubTester at the same level.
Also:

The null reference can always be cast to any reference type. In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.

thanks Ravish!

[Brian Lugo]

I am not sure if this will help:

public class TestPrintNull {
public static void main(String[] args) {
// This causes a CTE:
// "Reference to println is ambiguous. 
// It is defined in void println(java.lang.String) and void println(char[])."
// Meaning null can represent both java.lang.String and char[]
//System.out.println(null);
// This prints null correctly
System.out.println("" + null);
Object obj = null;
// This also prints null correctly and you
// do not need to have a concatenation operator
System.out.println(obj);

}

}