public static void main (String [] args) {
	final Display display = new Display ();
	final Shell shell = new Shell (display);
	shell.setText ("Lazy Tree");
	shell.setLayout (new FillLayout ());
	final Tree tree = new Tree (shell, SWT.BORDER);
	File [] roots = File.listRoots ();
	for (int i=0; i<roots.length; i++) {
		TreeItem root = new TreeItem (tree, 0);
		root.setText (roots [i].toString ());
		root.setData (roots [i]);
		new TreeItem (root, 0);
	}
	tree.addListener (SWT.Expand, new Listener () {
		@Override
		public void handleEvent (final Event event) {
			final TreeItem root = (TreeItem) event.item;
			TreeItem [] items = root.getItems ();
			for (int i= 0; i<items.length; i++) {
				if (items [i].getData () != null) return;
				items [i].dispose ();
			}
			File file = (File) root.getData ();
			File [] files = file.listFiles ();
			if (files == null) return;
			for (int i= 0; i<files.length; i++) {
				TreeItem item = new TreeItem (root, 0);
				item.setText (files [i].getName ());
				item.setData (files [i]);
				if (files [i].isDirectory()) {
					new TreeItem (item, 0);
				}
			}
		}
	});
	Point size = tree.computeSize (300, SWT.DEFAULT);
	int width = Math.max (300, size.x);
	int height = Math.max (300, size.y);
	shell.setSize (shell.computeSize (width, height));
	shell.open ();
	while (!shell.isDisposed ()) {
		if (!display.readAndDispatch ()) display.sleep ();
	}
	display.dispose ();
}