Add a popup menu to JTree in Java

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import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Window;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JPopupMenu;
import javax.swing.JScrollPane;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreeNode;
import javax.swing.tree.TreePath;

public class Main extends JPanel {

  DefaultMutableTreeNode root = new DefaultMutableTreeNode("java2s.com", true),
      node1 = new DefaultMutableTreeNode("node 1", true), node2 = new DefaultMutableTreeNode(
          "node 2", true), node3 = new DefaultMutableTreeNode("node 3", true);

  MyJTree tree = new MyJTree(root);

  public Main() {
    root.add(node1);
    node1.add(node2);
    root.add(node3);
    setLayout(new BorderLayout());
    add(new JScrollPane((JTree) tree), "Center");
  }
  public static void main(String s[]) {
    JFrame frame = new JFrame("Tree With Popup");
    Main panel = new Main();

    frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    frame.setBackground(Color.lightGray);
    frame.getContentPane().add(panel, "Center");

    frame.setSize(panel.getPreferredSize());
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
}
class MyJTree extends JTree implements ActionListener {
  JPopupMenu popup = new JPopupMenu();
  JMenuItem mi = new JMenuItem("Insert a children");

  MyJTree(DefaultMutableTreeNode dmtn) {
    super(dmtn);    
    mi.addActionListener(this);
    mi.setActionCommand("insert");
    popup.add(mi);
    mi = new JMenuItem("Remove this node");
    mi.addActionListener(this);
    mi.setActionCommand("remove");
    popup.add(mi);
    addMouseListener(new MouseAdapter() {
      public void mouseReleased(MouseEvent e) {
        if (e.isPopupTrigger()) {
          popup.show((JComponent) e.getSource(), e.getX(), e.getY());
        }
      }
    });

  }

  public void actionPerformed(ActionEvent ae) {
    DefaultMutableTreeNode dmtn, node;

    TreePath path = this.getSelectionPath();
    dmtn = (DefaultMutableTreeNode) path.getLastPathComponent();
    if (ae.getActionCommand().equals("insert")) {
      node = new DefaultMutableTreeNode("children");
      dmtn.add(node);
      ((DefaultTreeModel) this.getModel()).nodeStructureChanged((TreeNode) dmtn);
    }
    if (ae.getActionCommand().equals("remove")) {
      node = (DefaultMutableTreeNode) dmtn.getParent();
      int nodeIndex = node.getIndex(dmtn);
      dmtn.removeAllChildren();
      node.remove(nodeIndex);
      ((DefaultTreeModel) this.getModel()).nodeStructureChanged((TreeNode) dmtn);
    }
  }
}