Directory explorer gets NPE

Question

The following code is an adapted version of FileTreeModel that displays only directories. However. I receive a null pointer exception when it tries to access a folder that requires administrator privileges. An example might look like

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)

To try and limit this I added an argument that only adds visible folders to the JTree, however, that does not catch them all. Is there a way to exclude all folders that require administrator privileges to be explored?

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() can return null if given File is not a directory or not readable. See JavaDoc.

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