Doubt.......null?

[Shweta R Dhaneshwar]

Consider this code,I thought that this code won't compile since I was wondering how a null can be added to a null.This code compiles just fine.Next I thought that may be this will give a NullPointerException when run....but to my surprise the o/p is "nullnull".please explain .

 
 
class Test {

        public static void main(String args[]) {
String s1 = null;
String s2 = s1 + null;
 
System.out.println(s2);
}
}
 

[Siddhartha Bhattacharya]

operator overloading. string concatenation.

[Philip Heller]

The statement

String s2 = s1 + null;

is equivalent to

StringBuffer sb = new StringBuffer();
sb.append(s1);
sb.append(null);
s2 = sb.toString();

StringBuffer's append() method can handle null args.

[rakesh damkondwar]

Hi Philip,
The following code,

StringBuffer sb = new StringBuffer();
sb.append(s1);
sb.append(null);
s2 = sb.toString();

should give compile time error "saying ambigutiy in calling method .append(char []) on StringBuffer".

Thanks,
~Rakesh

[Mohit Agarwal]

Hi ,

Any one please clarify this .
I feel that adding a null reference with another null reference should return a null.

I am perplexed .

Tx in advance ,

Mohit .

[Tony Morris]

What's the hard bit to understand?
If in doubt, consult the SDK source - it becomes quite evident.
Particularly the String.valueOf(Object) method.

[Animesh Shrivastava]

Hey All,
This is the sequence how the code runs:
1) It does a String.valueOf(null)--> If the null is passed as an argument, literal string "null" is returned
2) Then new StringBuffer("null") is called
3) Finally on the above created StringBuffer object append("abc") method is run.
The final output is a SringBuffer object having contents as "nullabc".

4) At the end toString() is called to return a new String object.

So,
a += "abc" ==> a = (new StringBuffer((String.valueOf(null)).append("abc")).toString();

Hope u got this

But there are some slight changes the way the program runs depending on the variables,
---------------------------
Now if u have

 String a = null;
 String b = null;
 a += b;
 System.out.println("String val:" +a); 

The way it runs is like this
a = (new StringBuffer((String.valueOf(a)).append(String.valueOf(b))).toString();

----------------------------

Now if u have something like this

 String a = "abc";
 String b = null;
 a = "abc" + b; 

The simplified statement is:
a = (new StringBuffer("abc").append(b)).toString();

-----------------------------------------------------------
In the method append() of StringBuffer u have something like this:

 if (str == null) {   
     str = String.valueOf(str);
 }

Hopw it clears your doubt

Please also look here, i have copied from this post

Concatenation

[Jim Yingst]

[Rakesh]: ...should give compile time error "saying ambigutiy in calling method .append(char []) on StringBuffer".

A minor error in the code provided - StringBuffer has many, many overloads, and a null argument doesn't have enough info for the compiler to tell which one is meant. Just replace

    sb.append(null);

with

    sb.append(String.valueOf(null));

or insert a cast to (Object) or String) remove the ambiguity.

[Mohit]: I feel that adding a null reference with another null reference should return a null.

That's not what happens. Since one of the operands has the declared type String, the + operator indicates string concatenation. The rules for string concatenation are described in JLS 15.18.1. The relevant part here is: "If the reference is null, it is converted to the string 'null'"

[Mohit Agarwal]

Thanks to all of you for clearing the doubt and providing such a lucid explanation .

Bye.

[Philip Heller]

Oops, Rakesh is right. I meant to say

  
StringBuffer sb = new StringBuffer();
sb.append(s1);
sb.append((Object)null);
s2 = sb.toString();

[Nischal Tanna]

Originally posted by Shweta R Dhaneshwar:
Consider this code,I thought that this code won't compile since I was wondering how a null can be added to a null.This code compiles just fine.Next I thought that may be this will give a NullPointerException when run....but to my surprise the o/p is "nullnull".please explain .

 
 
class Test {

        public static void main(String args[]) {
String s1 = null;
String s2 = s1 + null;
 
System.out.println(s2);
}
}
 

Always remember Object+String is always a String

[Nik Raut]

That is the most simple and efficient way to put it...

object + String = String

[rakesh damkondwar]

Hi Phillip,

Phillip - "I meant to say

StringBuffer sb = new StringBuffer();
sb.append(s1);
sb.append((Object)null);
s2 = sb.toString(); "

I could not understand one thing in your above code, does the compiler explicitely type casts null to Object when calling append() method on StringBuffer. If not then how does above case would be handled by compiler, as when i try to pass null to append() method it throws compile time error.

Thanks,
~Rakesh

[Shweta R Dhaneshwar]

Thank you all for your efforts and time.Now its clear to me how it works

[Chitra AP]

Hi Swetha,

I didn't get it correctly. Could you please explain? Thanks.