Method overloading and passing null [duplicate]

Question

I have the following 2 methods overloaded in a class :

public class Test{

  public static void main(String []args) throws ParseException{
       Test t = new Test();
      t.testMethod(null);
  }

  public void testMethod(Object o){
    System.out.println("Object");
  }

  public void testMethod(String s){
    System.out.println("String");
  }
}

When I invoke the method testMethod it print "String".

When I add one more overloaded method :

public void testMethod(StringBuilder sb){
    System.out.println("String");
}

It throws me compiler error : The method testMethod is ambigous for type Test..

All this happens when I invoke the method with null

My questions are :

1. Why it prints String and not Object?
2. Why is there compilation error on adding third method?

Answer 1

Method resolution works by selecting the most specific method that is applicable for the given arguments. In your first case, String is more "specific" than Object (since it is a subclass of Object) and so the String method is chosen.

When you add another method that takes a StringBuilder, both that and the String method are equally "specific" (they are both direct subclasses of Object), so there is an ambiguity as to which should be chosen, hence the error.