Directory tree in Java

The most relevant classes in this case are the javax.swing.JTree, javax.swing.tree.TreeModel, javax.swing.tree.MutableTreeNode, javax.swing.tree.TreePath and javax.swing.event.TreeSelectionListener.

The JTree will take care of the GUI side, while the TreeModel holds the data wrapped in TreeNodes. You migth recognice this as a MVC (Model View Controller) pattern. As for the other two classes, they are farily descriptive; TreePath expresses a path between two hierarical nodes in the tree while the TreeSelectionListener listens for node selection.

A typical usage of the javax.swing.tree classes was seen in the week 43 2002 excercise. Typically, you create a root node, optionally add sub nodes, create a model with a reference to the root node and add the model to the JTree.

TreeNode root = new DefaultMutableTreeNode("Root");
TreeModel treeModel = new DefaultTreeModel(root);
JTree tree = new JTree( treeModel );

In this example the root note will be one called Computer, or My Computer as the friendly term goes. If we are dealing with a Unix or Linux OS, this might be unnecessary, since there is only one root file system, however on Windows there are usually several drives with a seprate root name. In this article we handle this by creating the abstract Computer root node and add any concret root files systems as children to this.

MutableTreeNode root = 
  new DefaultMutableTreeNode("Computer");

//get all nodes for top file systems or disks
File roots[] = File.listRoots();

int i = 0;
for(File f : roots) {
    String name = f.getAbsoluteFile().toString());
    DefaultMutableTreeNode node = 
      new DefaultMutableTreeNode(name);
    root.insert(node, i++);
}

The second task is to capture the selection event and expand the node with subdirectories if they exisit. In this example we have an inner class which implements TreeSelectionListener and gets a TreeSelectionEvent, which contains the selected node and the tree path to it. This can be retrived as shown below.

TreePath path = e.getNewLeadSelectionPath();

DefaultMutableTreeNode curNode = 
  (DefaultMutableTreeNode)path.getLastPathComponent();

From the TreePath we can construct the file system path by looking at the data object in each node and concat them with the correct file separator

Object nodes[] = path.getPath();
String dir = "";

for (int i = 1; i < nodes.length; i++) {
  DefaultMutableTreeNode node = 
    (DefaultMutableTreeNode) nodes[i];
  dir += (String)node.getUserObject();
  dir += File.separator;
}

Finally, we need a mechanism for getting all sub-directories, if any, of the selected node and add them as sub-nodes. The first task can be accived very easily by implementing the java.io.FileFilter and using it in the File.listFiles() method. In the code below the path variable holds the complete file path of the node to expand.

DirectoryFileFilter filter = 
  new DirectoryFileFilter();
        
File dir = new File(path);
File subDirs[] = dir.listFiles(filter);

The DirectoryFileFilter only has to implement one simple method:

public boolean accept(File f) {
  return f.isDirectory();
}

The complete example can be downloaded below. Note that this code will be part of an open source application, and is licenced under LGPL.

FileTreePanel.java

DirectoryFileFilter.java