About null-reference

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From https://codegym.cc/quests/lectures/questcore.level05.lecture03 : "In the second case, there won't be any compilation errors and void print(String s) will be called, which is somewhat not obvious:

public class Cat {

    public static void print(Object o) {
        System.out.println(o);
    }
    public static void print(String s) {
        System.out.println(s);
    }

    public static void main(String[] args) {
        Cat.print(1);
        Cat.print(null);
    }
}

Why the print(String s) method will be called? The null-reference can not belong to the Object, only to the String or the Integer?!... In the previous example in the lecture we needed to do this: Cat.print((String)null); In this example why don't we need to do the same?

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Nouser

Level 36 , Germany

7 March 2021, 08:39

useful

good questions and usually that kind of why can be found inside the JLS. Not really funny to read but sometimes enlightening. I had that moment with enums ;) So here from the JLS: 5.1.11. String Conversion Any type may be converted to type String by string conversion. ... If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Gellert Varga

Level 23 , Szekesfehervar, Hungary

7 March 2021, 14:12

Thank You! I looked that link, but I'm sorry, i wasn't able to understand my problem from it. "Any type may be converted to type String by string conversion. ... If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l). " - It's OK. But the question:

public class Cat {
    public static void print(Integer n) { // Integer is a reference-type, so it can be null
        System.out.println(n);
    }
    public static void print(String s) { // String is a reference-type, so it can be null
        System.out.println(s);
    }
    public static void main(String[] args) {
        Cat.print(null);
    }
}

In this case it's easily understandable, why we will get this compiler-error: "reference to print is ambiguous. both method print(Integer) in Cat and method print(String) in Cat match". But is this code below why not dubious?

public class Cat {
    public static void print(Object o) { // Object is a reference-type, so it can be null
        System.out.println(o);
    }
    public static void print(String s) { // String is a reference-type, so it can be null
        System.out.println(s);
    }
    public static void main(String[] args) {
        Cat.print(null);
    }
}

In this case, how can the JVM decide which method to call? Both methods would be able to receive a null reference.

Nouser

Level 36 , Germany

7 March 2021, 14:26

solution

You ask questions ;) If you want to hear my guess then String is a child of Object and the compiler is taking the more special but still fitting class. String and Integer on the other hand are on different inheritance lines so which path the compiler should take? Both are equal possibilities for the compiler. You need to make the decision. as a code experiment try that, it'll chose the more special class

public static void print(Number n) {
    System.out.println("Number " + n);
}
public static void print(Integer n) {
    System.out.println("Integer " + n);
}
public static void main(String[] args) {
    Cat.print(null);
}

Gellert Varga

Level 23 , Szekesfehervar, Hungary

7 March 2021, 15:24

This was a very logical and understandable line of thought. I did code experiments and it really seems like the JVM is behaving exactly that way. So, now I really got the answer:) Thank You, Nouser!