Справочник исследователь получает NPE

Question

Следующий код - адаптированная версия FileTreeModel, которая отображает только каталоги. Однако. Я получаю исключение нулевого указателя, когда он пытается получить доступ к папке, требующей прав администратора. Примером может выглядеть

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at DirectoryExplorer$FileTreeModel.listDirectories(DirectoryExplorer.java:112) 
    at DirectoryExplorer$FileTreeModel.getChildCount(DirectoryExplorer.java:79) 
    at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(Unknown Source) 
    at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(Unknown Source) 
    at javax.swing.tree.VariableHeightLayoutCache.ensurePathIsExpanded(Unknown Source) 
    at javax.swing.tree.VariableHeightLayoutCache.setExpandedState(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI.updateExpandedDescendants(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI$Handler.treeExpanded(Unknown Source) 
    at javax.swing.JTree.fireTreeExpanded(Unknown Source) 
    at javax.swing.JTree.setExpandedState(Unknown Source) 
    at javax.swing.JTree.expandPath(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI.handleExpandControlClick(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI.checkForClickInExpandControl(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(Unknown Source) 
    at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(Unknown Source) 
    at java.awt.Component.processMouseEvent(Unknown Source) 
    at javax.swing.JComponent.processMouseEvent(Unknown Source) 
    at java.awt.Component.processEvent(Unknown Source) 
    at java.awt.Container.processEvent(Unknown Source) 
    at java.awt.Component.dispatchEventImpl(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Window.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$500(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source) 

Чтобы попытаться ограничить это я добавил аргумент, что только добавляет видимые папки в JTree, однако, что не поймать их всех. Есть ли способ исключить все папки, требующие изучения прав администратора?

MCVE/SSCCE

import java.awt.GridLayout; 

import java.io.File; 

import java.util.Arrays; 
import java.util.ArrayList; 

import javax.swing.JFrame; 
import javax.swing.JPanel; 
import javax.swing.JScrollPane; 
import javax.swing.JTree; 
import javax.swing.tree.TreeModel; 


@SuppressWarnings("serial") 
public class DirectoryExplorer extends JFrame { 
    private DirectoryExplorer() { 
     super("Directory Explorer"); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     setLayout(new GridLayout(1, 1)); 
     createPanel(); 
     pack(); 
     setVisible(true); 
    } 


    private void createPanel() { 
     JPanel panel = new JPanel(new GridLayout(1, 1)); 

     TreeModel model = new FileTreeModel(new File("C:/")); 
     JTree tree = new JTree(model); 
     tree.setShowsRootHandles(true); 
     tree.collapseRow(0); 

     panel.add(new JScrollPane(tree)); 
     getContentPane().add(panel); 
    } 

    public static void main(String[] args) { 
     new DirectoryExplorer(); 
    } 

    private static class FileTreeModel implements TreeModel { 

     private File root; 

     public FileTreeModel(File root) { 
      this.root = root; 
     } 

     @Override 
     public void addTreeModelListener(javax.swing.event.TreeModelListener l) { 
      //do nothing 
     } 

     @Override 
     public Object getChild(Object parent, int index) { 
      File f = (File) parent; 
      return listDirectories(f)[index]; 
     } 

     @Override 
     public int getChildCount(Object parent) { 
      File f = (File) parent; 
      if (!f.isDirectory()) { 
       return 0; 
      } else { 
       return listDirectories(f).length; 
      } 
     } 

     @Override 
     public int getIndexOfChild(Object parent, Object child) { 
      File par = (File) parent; 
      File ch = (File) child; 
      return Arrays.asList(listDirectories(par)).indexOf(ch); 
     } 

     @Override 
     public Object getRoot() { 
      return root; 
     } 

     @Override 
     public boolean isLeaf(Object node) { 
      return false; 
     } 

     @Override 
     public void removeTreeModelListener(javax.swing.event.TreeModelListener l) { 
      //do nothing 
     } 

     @Override 
     public void valueForPathChanged(javax.swing.tree.TreePath path, Object newValue) { 
      //do nothing 
     } 

     private File[] listDirectories(File path) { 
      ArrayList<File> arrayList = new ArrayList<File>(); 
      for(File temp : path.listFiles()) { 
       if(temp.isDirectory() && !temp.isHidden()) 
        arrayList.add(temp); 
      } 

      return arrayList.toArray(new File[0]); 
     } 
    } 
} 

Answer 1

File.listFiles() может вернуться null, если данный файл не является каталогом или не читается. См. JavaDoc.

if(temp.isDirectory() && !temp.isHidden() && temp.listFiles()!=null)