title: java.lang.NullPointerException when scanning directories
body: <p>I have a very simple method that scans a directory structure to perform a check. The scanning looking like this:</p>
  
  <pre><code>File file = new File(initpath);
  
  for(File hex : file.listFiles(new HexagonNameFilter())) {
  
      for(File wall : hex.listFiles()) {
  
          for(File shelf : wall.listFiles()) {
  
              for(File book : shelf.listFiles()) {
  
                  // Perform some actual work
              }
          }
       }
   }
  </code></pre>
  
  <p>The method is called lots of times during the execution of the program. </p>
  
  <p>Inconsistently (meaning, at some unpredictable point in the scanning process), I get a java.lang.NullPointerException with the stack trace pointing at one of the for statements (which one it is is also inconsistent). This is not enlightening. I was thinking of passing FilenameFilters to the three listFiles() calls, but can't see how that would help the issue.</p>