When I pass a null, which overloaded function will java run

Question

So i can pass nulls into functions in java. I can also overload functions in java. But consider the following

public static void main(String ... args){
    doStuff(null);
}

public static void doStuff(String s){
      solveWorldHunger();
}

public static void doStuff(Integer i){
      nukeCanada();
}

public static void nukeCanada(){
 System.out.println("NO!");
}

public static void solveWorldHunger(){
  System.out.println("YAY!");
}

The previous program will always print out YAY no matter what the order of the sorce code...

Can anyone shed some light onto why the jvm consitently decides to run the solveWorldHunger function over the nukeCanada function`?

Answer 1

It's quite clear to the compiler that you can't store null in a primitive int. So, it decides to go with the method which has String as argument.

However, when you change your 2nd method to take Integer as argument, then you will get compiler error. Because both of them are eligible to be invoked with null argument. So, there will be ambiguity.

So, try changing your 2nd method: -

public static void doStuff(String s){
      solveWorldHunger();
}

public static void doStuff(Integer i){  // Integer instead of `int`
      nukeCanada();
}

And invoke it on null. You will see the compiler error.

So, in that case, you can invoke the appropriate method using typecasting like this: -

doStuff((Integer)null);  // Calls Integer version
doStuff((String)null);   // Calls String version

---

Also, note that, if you have your 2nd method take a parameter that is a super type of String, like Object, then there would be no ambiguity, because compiler will now choose the most specific one, i.e., String argument.

So, ambiguity only occurs, when you have two methods with types that are not in the same inheritance hierarchy.

Answer 2

Only your doStuff(String) method can take a NULL argument... the primitive int can never be null. If you had a second method overload that passed an object, you would no longer be able to pass null as a parameter for that method.