Java method dispatch with null argument

Why does it (apparently) make a difference whether I pass null as an argument directly, or pass an Object that I assigned the value null?

Object testVal = null;
test.foo(testVal);    // dispatched to foo(Object)
// test.foo(null);    // compilation problem -> "The method foo(String) is ambiguous"   

public void foo(String arg) { // More-specific
    System.out.println("foo(String)");
}

public void foo(Object arg) { // Generic
    System.out.println("foo(Object)");
}

In other words, why is the (commented-out) second call to foo(...) not dispatched to foo(Object)?

Update: I use Java 1.6. I could compile Hemal's code without problems, but mine still doesn't compile. The only difference I see is that Hemal's methods are static while mine are not. But I really don't see why this should make a difference...?

Update 2: Solved. I had another method foo(Runnable) in my class, so the dispatcher couldn't unambiguously select the single most specific method. (See my comment in Hemal's second answer.) Thanks everyone for your help.

Solution

Which version of Java are you using? With 1.6.0_11 the code (pasted below) compiles and runs.

I am sure its obvious why foo(testVal) goes to foo(Object).

The reason why foo(null) goes to foo(String) is a little complex. The constant null is of type nulltype, which is a subtype of all types. So, this nulltype extends String, which extends Object.

When you call foo(null) compiler looks for the overloaded method with most specific type. Since String is more specific then Object that is the method that gets called.

If you had another overload that was as specific as String, say foo(Integer) then you would get a ambiguous overload error.

class NullType {

  public static final void main(final String[] args) {
    foo();
  }

  static void foo()
  {
    Object testVal = null;
    foo(testVal);    // dispatched to foo(Object)
    foo(null);    // compilation problem -> "The method foo(String) is ambiguous"   
  }

  public static void foo(String arg) { // More-specific
    System.out.println("foo(String)");
  }

  public static void foo(Object arg) { // Generic
    System.out.println("foo(Object)");
  }

}