title: NullPointerException while reading multiple files from a directory
body: <p>When I run the following piece of code, I'm getting a NullPointerException. This method is used to scan a directory and retrieve all files(.java) in it and then count the NLOC and NLC. </p>
  
  <p>So here is the code :</p>
  
  <pre><code>public void listFilesForFolder(final File folder) throws IOException {
      List&lt;String&gt; lines;
      for (File fileEntry : folder.listFiles()) {  // &lt;&lt;--- exception here
        if (fileEntry.isDirectory()) {
          listFilesForFolder(fileEntry);
        } else {
          if (fileEntry.isFile()) {
            temp = fileEntry.getName();
            if ((temp.substring(temp.lastIndexOf('.') + 1, temp.length()).toLowerCase()).equals("java")) {
              fileName = folder.getAbsolutePath() + "\\" + fileEntry.getName();
              try {
                lines = getLines(fileName);
                totalCommentCount += calcComments(lines);
                totalLOC += calcLOC(lines);
  
              } catch (IOException e) {
                System.out.println("File does not exist!" + e);
              }
            }
          }
        }
      }
      StringBuilder output = new StringBuilder();
      output.append("Comment Lines :").append(totalCommentCount);
      output.append("\n");
      output.append("Lines of Code (LOC) :").append(totalLOC);
      txtDisplayResults.setText(output.toString());
   }
  </code></pre>
  
  <p>Here is the error message (I use netbeans as the IDE):</p>
  
  <pre><code>23-Jul-2013 13:17:53 scaj.GUI listFilesForFolder
  SEVERE: null
  java.lang.NullPointerException
      at scaj.GUI.listFilesForFolder(GUI.java:61)
      at scaj.GUI.listFilesForFolder(GUI.java:68)
      at scaj.GUI.bScanActionPerformed(GUI.java:326)
      at scaj.GUI.access$100(GUI.java:17)
      at scaj.GUI$2.actionPerformed(GUI.java:235)
      at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
      at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
      at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
      at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
      at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
      at java.awt.Component.processMouseEvent(Component.java:6038)
      at javax.swing.JComponent.processMouseEvent(JComponent.java:3260)
      at java.awt.Component.processEvent(Component.java:5803)
      at java.awt.Container.processEvent(Container.java:2058)
      at java.awt.Component.dispatchEventImpl(Component.java:4410)
      at java.awt.Container.dispatchEventImpl(Container.java:2116)
      at java.awt.Component.dispatchEvent(Component.java:4240)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
      at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
      at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
      at java.awt.Container.dispatchEventImpl(Container.java:2102)
      at java.awt.Window.dispatchEventImpl(Window.java:2429)
      at java.awt.Component.dispatchEvent(Component.java:4240)
      at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
      at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
      at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
  </code></pre>