Object vs String Method [duplicate]

Question

Possible Duplicate:
Overloaded method selection based on the parameter’s real type
How is an overloaded method choosen when a parameter is the literal null value?

When I execute the code below, I get the following output:

Method with String argument Called ..."

Why?

public class StringObjectPOC {

    public static void test(Object o)   {
        System.out.println("Method with Object argument Called ...");
    }
    public static void test(String str){
        System.out.println("Method with String argument Called ...");
    }
    public static void main(String[] args) {
        StringObjectPOC.test(null);
    }
}

Answer 1

The null in the call matches both the test() signatures. You have to cast the null to ensure it calls one or the other.

Answer 2

A similar example is from Java Puzzles if I recall correctly.

null can be of String type or Object type. But the JVM will always choose a more accurate method.

In this case, String is more accurate then Object. (a String is an Object, but an Object might not be a String).

It is not a good habit to write code like this. Try to cast parameters to match your desired method, like

public class StringObjectPOC {

    public static void test(Object o)   {
        System.out.println("Method with Object argument Called ...");
    }
    public static void test(String str){
        System.out.println("Method with String argument Called ...");
    }
    public static void main(String[] args) {
        StringObjectPOC.test((Object)null);
    }
}

Answer 3

I tried this:

Test2.class

public class Test2{}

Test3.class

public class Test3 extends Test2{

}

Test.class

public class Test{

public static void print(Object obj){
    System.out.println("Object");
}

public static void print(Test2 obj){
    System.out.println("test 2");
}

public static void print(Test3 obj){
    System.out.println("Test 3");
}


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

}

it printed out Test 3

Just like in your scenario, this means that if a method is overloaded (and when null is passed), it recognizes the method which has the child-most parameter.

Object->Test->Test2

or in your case:

Object->String

Answer 4

Java tries to find the most specific method possible to call. String is a subclass of object, so Java will defer to the String method whenever it can. null is a perfectly acceptable value for either an Object or a String, but since a String is more specific than an Object, Java defers to the String method because it is more precise.