JTree with JLabel nodes containing HTML code throws exception when expanding nodes

Question

Your getting a ClassCastException because getUserObject() will initially return a String, which can't be casted to a JLabel.

You can just use setText(...) to replace the text. Something like this

DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
String text = ((String) node.getUserObject());
String newText = "<html><FONT COLOR=ORANGE size=\\\"2\\\"><B>" + text + "</B></FONT></html>";
setText(newText);

return this;

enter image description here

import java.awt.Component;
import javax.swing.JFrame;
import javax.swing.JTree;
import javax.swing.SwingUtilities;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeCellRenderer;

public class TreeCellRender {

    JTree tree = new JTree();

    public TreeCellRender() {
        DefaultTreeCellRenderer renderer = new DefaultTreeCellRenderer() {
            @Override
            public Component getTreeCellRendererComponent(JTree tree,
                    Object value, boolean sel, boolean expanded, boolean leaf,
                    int row, boolean hasFocus) {
                super.getTreeCellRendererComponent(tree, value, sel, expanded,
                        leaf, row, hasFocus);

                DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
                String text = ((String) node.getUserObject());
                String newText = "<html><FONT COLOR=ORANGE size=\\\"2\\\"><B>" + text + "</B></FONT></html>";
                setText(newText);
                return this;
            }
        };

        tree.setCellRenderer(renderer);

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(tree);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new TreeCellRender();
            }
        });
    }
}

Also, if all you wanted to do was set the font and color, you could have easily just done this in your method

public Component getTreeCellRendererComponent(....) {
    ....
    setFont(new Font("impact", Font.PLAIN, 20));
    setForeground(Color.GREEN);
    return this;
}

enter image description here

UPDATE

Another way proposed by the original poster is this

Another way: Create the tree with strings nodes containing the html code. The renderer will be like that:

public class MyTreeCellRenderer extends DefaultTreeCellRenderer {
    DefaultTreeCellRenderer defaultTreeCellRenderer=new DefaultTreeCellRenderer();

    @Override
    public Component getTreeCellRendererComponent(JTree tree,Object value, boolean sel, boolean expanded, boolean leaf,int row, boolean hasFocus) {
        super.getTreeCellRendererComponent(tree, value, sel, expanded, leaf, row, hasFocus);

        DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
        String text = (String)node.getUserObject();
        JTextPane textPane = new JTextPane();
        textPane.setContentType("text/html");
        textPane.setText(text);
        return textPane;
    }
}