What is the meaning of null?

Question

String s="";
while ((strLine = br.readLine()) != null)   {
    s=s.concat(strLine);

When I used this code I get the string I expect from the file .. But if I used

String s = null;

What I get is null as the result of the s string. Can anyone explain me the reason for this?

I think you should be getting `NPE` when using `String s = null;` — Sankalp, Jul 19 '13 at 15:46
@Sankalp: Yup, me too - I suspect the real code uses the string concatenation operator. — Jon Skeet, Jul 19 '13 at 15:46
BTW concatenating lines this way will put all the text on one line without a separator. — Peter Lawrey, Jul 19 '13 at 15:47
Have a look to this http://stackoverflow.com/questions/2707322/what-is-null-in-java — tokhi, Jul 19 '13 at 15:51

score 9 · Answer 1 · answered Jul 19 '13 at 15:44

Firstly, I suspect that's not your code - or it would throw a NullPointerException. I suspect you've actually got:

s = s + strLine;

After that, it's very simple - concatenating any string with a null String reference will give you null:

String x = null;
String y = x + "a";
System.out.println(y); // nulla

From section 15.18.1 of the JLS (string concatenation):

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Note that your code is extremely inefficient at the moment - you should be using a StringBuilder.

1 Answers1