JTree overrides getToolTipText(MouseEvent) and forwards the call to the renderer, which is good, but if the renderer doesn't have a tooltip it should fallback to that of super.getToolTipText. An implication of the current code is that setToolTipText on a JTree doesn't work.

Here's a test case:

import javax.swing.*;

public class Test2 {
    public static void main(String[] args) {
        JTree tree = new JTree();
        tree.setToolTipText("TEST");
        JFrame frame = new JFrame("TEST");
        frame.getContentPane().add(new JScrollPane(tree));
        frame.pack();
        frame.show();
    }
}