Null pointer not found

[Swamy Nathan]

Ok somebody explain this-

 
public class Test
{

public static void main(String[] args) 
{
System.out.println("1"+null+2);//1
System.out.println(null+"3"+2);//2
//System.out.println(null+2+"3");//not ok//3
//System.out.println(1+null+"2");//not ok//4
String s=null;//5
s+="a";//6
System.out.println(s);//7
s=null;//8
//s=s.concat("a");//not ok- throws java.lang.NullPointerException//9
//System.out.println(s);//10
String s1=null;//11
String s2=null;//12
String s3=null;//13
Integer i=null;//14
s3=s+s1+s2+i;//15
System.out.println(s3);//16



}

}

output:

 
1null2
null32
nulla
nullnullnullnull 

[john von]

explain what?

[Swamy Nathan]

If the following throws java.lang.NullPointerException
s=null;
s=s.concat("a");

How come this
("1"+null+2) does not?

And also in other places.

Am I correct in thinking this is because internally a StringBuffer is used in all cases and stringBuffer.append(null) just adds the text null.
Anyway I thought it was interesting because I was expecting a nullpointer to be thrown and it was not.