Java Iterator in for-loop NullPointerException

Question

I do not understand why a java.lang.NullPointerException is thrown with the following code (at this line: iterator = chapterKeywords[z].iterator();):

Iterator<String> iterator;

        for (int z = 0; z < chapterKeywords.length; z++) {

            try {

                iterator = chapterKeywords[z].iterator();
                //exception thrown here after first iteration (i.e., when z = 1) and subsequent ones

                while (iterator.hasNext()) {

                    System.out.println(iterator.next());

                }

            } catch (Exception e) {
               ;
            }

        }

Although I get the NullPointerException, the code fully executes. FYI, chapterKeywords is a LinkedList Array where each array element is a LinkedList (e.g., singleChapterKeywords).

LinkedList<String>[] chapterKeywords = new LinkedList[numFiles];
LinkedList<String> singleChapterKeywords = new LinkedList<String>();

Although the program seems to run fine, I would like to understand why the NullPointerException is thrown as I would like to completely eliminate it. Basically, the exception is thrown at the start of each iteration in the for-loop. I've modified the code to create a new iterator inside the loop, create an array of iterators, etc but they all yield the same result: NullPointerException at iterator = chapterKeywords[z].iterator();.

Any help or insight into eliminating this exception is greatly appreciated.

So, here are my questions:

Why is there a NullPointerException thrown at each subsequent iteration when the iterator variable is being assigned to a new iterator?

How can I eliminate this exception?

Once again, TIA!

Update: I thought that the LinkedList's were properly populated and worked fine, but, alas, this was not the case! Thank you all for your immeasurable help!

Thanks. Most certainly, chapterKeywords[z] was all hosed up! — Luis
– Luis, Commented Aug 8, 2012 at 21:55
To make matters worse I had accidentaly included the above loop into another loop (missed it by a bracket!), resulting in the squaring of the number of iterations (numFiles^2 instead of numFiles!)... — Luis
– Luis, Commented Aug 9, 2012 at 0:15

Andreas Dolk · Accepted Answer · 2012-08-08 20:28:46Z

5

Looks like you've populated the lists but missed to add (some of) them to the array.

This will reveal your problem:

for (int z = 0; z < chapterKeywords.length; z++) {
   if (chapterKeywords[z] == null) {
      System.out.println("No list found at array index " + z);
      continue;
   }
   iterator = chapterKeywords[z].iterator();
   while (iterator.hasNext()) {
      System.out.println(iterator.next());
   }
}

edited Aug 8, 2012 at 20:28

answered Aug 8, 2012 at 20:20

Andreas Dolk

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1 Comment

Luis Over a year ago

Many thanks! Your code helped me understand that the problem layed with chapterKeywords[z] and not the iterator. Once again, many thanks for your help!

Rocky Pulley · Accepted Answer · 2012-08-08 20:21:57Z

1

chapterKeywords[z] is null. It definitely exists or you would get an out of bounds error, but you didn't actually ever call something like chapterKeywords[z] = new String()..

answered Aug 8, 2012 at 20:21

Rocky Pulley

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1 Comment

Luis Over a year ago

Thanks. Indeed, chapterKeywords[z] was all hosed up!

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