the only possible things that could be null and cause this exception are
s (which is clearly not the case, since s.length does not throw a NPE),
or s[j].  Are you SURE that s[j] isn't null?  Because that's what it looks
like is the case.

You did not copy/paste the stack trace.  Did exception really occur
inside nextChar()?

Hi, either way it is false. It must be coming from the class with which it
is in.

Michael, this is why you need to post more code and the stack trace. For
instance, you dont say what type the reference s is above...now we infer
from the way that its used that is a String[], but technically it could
be just about anything.

Also, without the actual error, there's no way we can be sure that
you're right.

I see you posted the error later, that is good. So let's look at it now:

Exception in thread "main" java.lang.NullPointerException
       at vertical.main(vertical.java:39)

What this error means is that line 39 of source file vertical.java
(which should be capitalized, along with the class name, by the way)
contains a null pointer that has a method called on it or field accessed
in it.

So in your code, if that is indeed line 39 of vertical.java, that means
the null pointer exception happens IN YOUR CODE. It can't possibly
happen in nextChar(), or else the error would say

"... NullPointerException at String.nextChar( ..."

Therefore, either s is null, which you know it isn't because you
accessed the length field one line above, or s[j] is null. Now you know
its in your code, not anybody elses. (By the way, code in the standard
library is unlikely to ever throw NPE on its own...as a beginner, never
assume that the library is wrong and you're right. You'll be wrong
99.99% of the time.)

Notice that whenever you evaluate s[j] (for j>0) you've only tested to
see if s[j-1] is null. This had misled you to thinking that s[j] is
non-null when you call line 39.

Please note in the future that it really helps to post a fully working
example. Take whatever you are having a problem with, and cut and paste
it into a test class. Then compile it and run it, and SHOW ALL OF YOUR
RESULTS. Here is an example:

===========================================

marks:~ mehaase$ cat Test.java
class Test {
public static void main(String args[]) {
 String s = "Hello, world!";
 String q = "test";

 q = null;
 
 System.out.println("is s null? = " + (s == null));
 System.out.println("s is \"" + s + "\"");
 System.out.println("s length = " + s.length());

 System.out.println("is q null? = " + (q == null));
 System.out.println("q length = " + q.length());
}
}
marks:~ mehaase$ javac Test.java
marks:~ mehaase$ java Test
is s null? = false
s is "Hello, world!"
s length = 13
is q null? = true
Exception in thread "main" java.lang.NullPointerException
       at Test.main(Test.java:13)

===========================================

You'll notice that I've shown the full source code (notice that the name
of the class is capitalized, you SHOULD do this), which is a compact
example I've used to make my point. I also showed how I compiled it, and
then the results of running it.

Note that this serves two purposes: 1) you generally answer your own
question by the time you've refactored the code like this. 2) it makes
it much easier for us to help you when you show us everything in a
concise, comprehensive manner.

Look at my error, it says NPE in Test.main()...that means the error is a
null reference IN MY CODE, not in length(). If it was an error in
length(), the error would say. Also, notice that I test if it is null
BEFORE I call a method on it. You test if it is null AFTER you call a
method on it, which is no better than not testing at all.

Learn those errors, learn how to read them, and learn how to make
effective posts here and you will do very well in your class.

ALSO -- GO TO YOUR OFFICE HOURS!! An hour spent one-on-one with a prof
will save days of writing posts here and waiting for responses.

I hope its clear that a lot of people here want to help you, I spent 10
minutes writing this response. But you've got to exert a little bit more
effort and meet us halfway.

mehaase(at)gmail(dot)com

Hi,

If you're using Eclipse, you can easily debug exactly which expression is giving
the exception:

1. Toggle a breakboint on that line.
2. Debug.
3. Evaluate each expression by selecting it en pressing <ctrl><alt>+'i'. You can
try

<s>  (should not be null, as s.length did not give an exception.)
<s[j]>
<s[j].nextChar()>

Greetings, Leon.