null String

[Nidhi Sar]

Why doesn't this code give NullPointerException:

String s = null + "";

s contains the string "null". How is this happening?

[Phungsuk Wangdu]

because whenever you use + with either operands as string,the result is a string

[Nidhi Sar]

Thanks James. I understand that it's true with all primitives & objects that "+ String" converts the whole thing to a String. But null just indicates no obeject, nothing. How does that get converted to the literal string "null".

  		String s = null + "";
		String s1 = "null";
		System.out.println(s.equals(s1));  //prints true
 

[Raju Champaklal]

well that is a rule in the jls...anything with + and null is converted to strings

[Phungsuk Wangdu]

when you are wrting
String s=null or "null"
the interprets it as String s=new String("null");
well if you want a null pointer exception use String h; and leave it uninitialised and perform any operation on it like toUpperCase();
then you will get null pointer exception


EDIT : ok my bad now i have corrected my error

[Raju Champaklal]

String s="null" ....doesnt mean String s=new String(null)...i pretty sure am right

[Phungsuk Wangdu]

HOW??

[Raju Champaklal]

let me check

[Nidhi Sar]

String s = new String(null); does not compile for me.

However, in the following code, it is clear that a null String is different from a String initialised as "null"

 String s1 = "null";
		String s2 = null;
		s1.concat("abc"); //No error, runs fine
		s2.concat("abc"); //NullPointerException
   

So I'm still wondering how String s = null + ""; becomes the literal "null"

[Raju Champaklal]

String s=new String(null);
this doesnt compile

[Raju Champaklal]

there are two overloaded contuctors for strings String(String) and String(StringBuffer)
when you call new String(null).this gives you an ambiguous error....
String s=null + "" gives null as anything with + is converted to a string literal

[Phungsuk Wangdu]

 class jeopardy{
public static void main(String ...h){
String s=null;
s=s+"key";
System.out.print(s);
s=s.toUpperCase();
System.out.print(s);
}
}   

well brother then how is this compiling??

[Phungsuk Wangdu]

i guess i have corrected it now

[maggie karve]

hey it only happens when we put the value null into string ..we can concat a null value with other string constants....but String s="null" and String s=null are 2 different things..one is having string value null with four characters and other is having null value.....
try doing this...

String s="null";
 System.out.println(s.charAt(3));-----u get answer at character 3 i.e.letter l
when u do this 
String s=null;
 System.out.println(s.charAt(3));

You get a null pointerException since you are trying to perform operation on a string having null value....i hope i am right....anybody correct me if i am wrong ..

[Raju Champaklal]

why wont this compile? what did i say that maked you thing that this wont compile? actually String(null) means String s=new String(null) and not String s=null; i was referring to the first one

[Phungsuk Wangdu]

so am I good now

[Raju Champaklal]

you are a good boy Mr.Carter

[Rajiv Chopra]

Nidhi Sar wrote:Why doesn't this code give NullPointerException:

String s = null + "";

s contains the string "null". How is this happening?

Hi I saw the discussion. I want to add something.

   import java.util.Locale;


public class TestString {
	static String s;
	public static void main(String[] args) {
		String k = s+"";
		System.out.println(k);
	}} 

null is a value which can be assigned to a reference object. It specifies the reference object has no object instance associated to it. In Java 'null' is a reserved word that can not be used as identifer and null means nothing or no value. 'null' is the default value all the uninitialized static & non-static reference objects.