Method overriding passing null object

Question

I am unable to understand why this program prints String

class AA {

    void m1(Object o) {
        System.out.println("Object ");
    }

    void m1(String o) {
        System.out.println("String ");
    }
}

public class StringOrObject {

    public static void main(String[] args) {
        AA a = new AA();
        a.m1(null);
    }
}

Please help me to understand how this works to print Sting rather than Object

Because it'll try to call the most-specific method, which in this case is String. — Dave Newton, May 18 '13 at 17:29
I am wondering what would be the outcome if a method like "void m1(Integer o)" would be declared in addition to the two? A compile-time error? — Dušan Rychnovský, May 18 '13 at 17:45

score 3 · Answer 1 · edited May 18 '13 at 17:43

3

Dave Newton's comment is correct. The call to the method goes to the most specific possible implementation. Another example would be:

class Foo {}
class Bar extends Foo {}
class Biz extends Bar {}


public class Main {

    private static void meth(Foo f) {
        System.out.println("Foo");
    }

    private static void meth(Bar b) {
        System.out.println("Bar");
    }

    private static void meth(Biz b) {
        System.out.println("Biz");
    }


    public static void main(String[] args) {
        meth(null); // Biz will be printed
    }
}

edited May 18 '13 at 17:43

Ernest Friedman-Hill

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answered May 18 '13 at 17:36

Emil H

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+1 for use of the `meth` function – The Kraken May 18 '13 at 17:38

score 0 · Answer 2 · answered May 18 '13 at 17:32

0

This will try and make the most specific call. Subclass object gets the preference which is String in this case.

answered May 18 '13 at 17:32

roger_that

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Method overriding passing null object

2 Answers2