Which constructor will be called in main

Hi frens,
Plzz...give ans to my qn following..
class A
{
String str;
A(A ob)
{
ob.str=str;
}

A(String s)
{
this.s=str;
}}

class Main
{
psvm(String a[])

{
A a=new A(null);
}
}

In the above code, which constructor will be called in main

Thanx & regards,
Sandhya

I guess,:confused:

It looks for a default constructor in the class 'A'. :eek:

As it is not their, it doesn't call any of the 2 constructors because they need a not null argument for its parameter. :D

Hi [B]sandhya[/B]. have you tried compiling and executing this code? It won't even compile! (I didn't try it in a compiler but it won't compile).

Simply because "null" is not an identifier/keyowrd. So, it will search for a variable names null which it won't find. So, it will throw an error saying "null" not found...

-Kalayama

Kalayama is right the above code will not compile.

Here is the reason:

we dont know which constructor to bind your A(null) call to.

Usually the most specific method/construcotr is called.

For example, if you had two constructors

A(Object b){}

and

A(String s){}

the A(String s) would be called for A(null) (Imagine null to be an empty object)

because String is child of Object and hence, is more specific.

But between A and String, one is not more specicifc than other.

There are other mishaps in the code though :)

-> You have defined the STring array passed to main as a. you can't redfeine a inside main again

-> inside the A(String s) you should have this.str = s and not the other way round

hope this helps

hi.............>>
kalyama what i think youn are using null in tyhe object in new operator and null is no keyword
so what i think it will go for default constructor what i think as i am just learning java

It will give the compilation error definetly..............

Sure A(String s) constructor will excecute .
bcoz null is not an object , is an string literal .

class A
{
String s;
A(A ob)
{
ob.s=s;
}

A(String s)
{
this.s=s;
}
}

class Main
{
public static void main(String a[])
{
A a1=new A(null);
}
}

During Copilation it will display the following Error:
Reference ro A is ambiguous, both method A(A) in A and method A(String) in A match.

if U use ,
this.str=s; it also produce compilation error.

hi
constructor with string argument will be called in main.

u can pass any type of argument in constructor and u can created its object in main method of class.

it will definetly give compilation error.If u r using new operator it supposed to invoke default constructor.Coz [U]null[/U] is not a parameter.its a string literal.

Hi ,

this won't cause any compile time or ru n time error.
it'll run successfully.

Sandhya,
First of all, you will get a compilation error, saying that the constructor A(a) is ambiguous if you change the constructor signature as A(Object a) then it will compile and the String constructor will be called, as in Java null is a special literal of the null type, it can be cast to any reference type but not to primitive type. As null cannot be casted to A it will throw ambiguous error. if you try to call A a1=new A((A)null); then it will compile and gets the A(A ob) constructor called.
If you change the Constructor signature as A(Object ob) then the String constructor will be called.
Remember null can be casted to any reference types.
Does this makes sense !
Enjoy java coding ! ! !

Compile time error will be caused.

hello,
Daisy deepa is correct it will give an compilation error stating : "reference to A is ambiguous, both method A(A) in A and method A(java.lang.String) in A match
A a=new A(null);"

here parameterized constructor will be called. because here null is a string . so second (Parameterized) constructor is called

it gives an error because the constructor does not pass null values

every1 has given their views abt null n ambiguity but.........
there is one more error
constructor cannot take class as its parameter
it can only take its reference .....

hi
In this code i think the 2nd constructor will be called bcos null is a string type.

Hi I have tried to compile this code,, but it gives me compilation error,,,,
The reason for that as my understanding is:
1. The order to invoke constructor is the most specific to generalize, hence "null" is not an identifier/keyowrd. and then non of constructor find that match ,, so throw an exception

[QUOTE=sutnarcha;8872]I guess,:confused:

It looks for a default constructor in the class 'A'. :eek:

As it is not their, it doesn't call any of the 2 constructors because they need a not null argument for its parameter. :D[/QUOTE]



[COLOR="Red"]sorry dear.. But ur wrong
It will not call any constructor it ll just give an compile time error...
it will say a message like constructor A is ambiguos[/COLOR]

[B][COLOR="Blue"]It will not cll any of the constructor ...
It ll just give u an Compile time error sying like[/COLOR]
[COLOR="Red"]constructor A is ambiguos[/B][/COLOR]

2nd contructor will be called

Compile time error will be caused.

No Constructor will be called.
Their must be an error..........

no constructor is invoked

what is the use of final class when we cannot not extend or import

it called default constructor but here not any constructor which accept null value, so it will give a compiler error.