null passed as parameter

[amal shah]

public class Test1{
static void method(String str){
System.out.println("String");
}
static void method(Object obj){
System.out.println("Object");
}
public static void main(String args[]){
method(null);
}
}

o/p is: String

when method(null) is called ...then why method(String) is invoked and why not method(Object)...the fundamentals are Object and String can both accept null values....unable to understand...help appreciated..

thanking you
amal shah

[Gowher Naik]

Because String is most specific then object.
Check the code below.

class Base{}
class SubClass extends Base{}
class SubClass1 extends SubClass{}
public class Child extends SubClass1{
    public static void main(String[] args) {
     new Child().method(new Child());
    }
    void method(Base b){//1st method
    System.out.println("Base Msg");
    }
    void method(SubClass b){//2nd method
    System.out.println("SubClass Msg");
    }
    void method(SubClass1 b){     //3rd method
    System.out.println("SubClass1 Msg");
    }
}
op-->SubClass1 Msg

i am passing Child object to method now all 3 methods can take Child object
so all three methods match.But 3rd method is choosen by compiler because it is more specific then other 2 methods.
if you will remove 3rd method from above code then 2nd method will match and out put will be
SubClass Msg
lly if you will remove 2nd and 3rd method from above code then only method will remain is first method and compile will make match with 1st method
which has only choice left for comiler.

[Daniel Bryant]

Hi,

I always think it's better to figure these problems out yourself rather than being explicitly told, and so at the risk of answering a question with a question, have a look at this code:

public class Test1{
    static void method(A obj){
        System.out.println("A");
    }
    
    //then maybe comment out this method
    static void method(B obj){
        System.out.println("B");
    }
    
    //comment out this method first to experiment
    static void method(C obj){
        System.out.println("C");
    }    
    
    public static void main(String args[]){
        method(null);
    }
}
 
class A {      
}
 
class B extends A {    
}
 
class C extends B {    
}
    

Compile and run it and see what output you get. Now experiment by commenting out the "method" methods above, one at a time and observe what happens to the output.

Can you see what's going on with the hierarchy of classes? Does this answer your question?

Best wishes,

Daniel

[Daniel Bryant]

looks like I was beaten to it!

Daniel

[wise owen]

Selecting the Most Specific Method

[amal shah]

thanks gowher,daniel and wise...that helped me to get my feet in water..now tryin to swim...

[marc weber]

This is also addressed in our SCJP FAQ.