Java method dispatch with null argument

Question

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

Answer 1

Sorry to use an answer, for a comment, but I need to post code that won't fit in comment.

@Yang, I am also able to compile and run the following. Can you post a complete code that compiles with one line commented such that if I uncomment that line it won't compile?

class NullType {

  public static final void main(final String[] args) {
    foo();
    new Test().bar(new Test());
  }

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

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


}


class Test
{
  void bar(Test test)
  {
    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)");
  }
}