Diagnosing a "hang" in an Iterator

Question

I have code that takes a list of ID numbers in an ArrayList The size of he ArrayList is 126. IN my code, I create an iterator on the list. For each id, I do a fetch from the database. After I get to 28 ids,the program simply stops. No error, no Exception. It just stops . I wanted to run JConsole but the box is not configured for that (or any X11-type stuff). I want to know what's goig on, like lowmemory. I have guessed it was memory and addd a <maxmemory="512m"/> to the java task of my ant script, but that seemed to amke no difference. How can I diagnose what's wrong? I use iterators all th time and never hit a situation like this. Here's the relevant code snippet:
Iterator idIter = studentIDs.iterator();
while(idIter.hasNext())
studentID = (String)idIter.next();
edu.apu.directory.APUPerson student = getStudent(studentID);
writePersonHeader(student);
counter++;
System.out.println("counter:"+counter);
studentIDs.size() just before this loop equals 126. When I run this, coutner never gets past 28. NO error. It just stops. Any suggestions? Thanks.

Answer 1

This code isn't complete. The hang isn't likely to be in the iterator. It's in the getStudent() method, I'll bet. Could an exception be thrown and you haven't really handled it? Look for an empty catch block somewhere that's swallowing an exception without telling you.
Thanks but that's not the case. The getStudent method uses a DAO that has been running in production for months and works just fine. There are no empty catch blocks:
private edu.apu.directory.APUPerson getStudent(String studentID)
try
student = (edu.apu.directory.APUPerson)personDAO.findAPUPerson(studentID);
catch (PersonNotFoundException e)
System.out.println("error getting student:"+ e.getMessage());
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
catch (LookupFailedException e)
System.out.println("error getting student:"+ e.getMessage());
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
return student;
What is of great interest is that if I copy the call to getStudent() out of the Iterator loop and hardcode an id in it, then the number of elements in the ArrayList that the Iterator woudl process went down by one, as though the application simply will not process over 28 calls to the DAO. I don't get any exceptions, and I never code empty catch blocks (bad programming practice, which can cause trouble, as you note).
I'd move that database code into a separate object and test it out separate from the rest of your code. It's called a DAO object. It ought to have a signature like this:
Good idea. Done long ago.
(I don't know what your edu.apu.directory.APUPerson class is. Perhaps that is your "Student".) Yes, it models a student.
If I were running this app, I'd get into a debugger and step through the code until I figured out exactly where the error was occurring. Shouldn't be too hard to do.
I"ll see if I can get that to work, but I'm dubious of it working 28 times and tehen telling me what's wrong the 29th, if it's not a Java error. Thanks.