Java Swing How to - Build a JTree dynamically

Question

We would like to know how to build a JTree dynamically.
Answer

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.io.File;
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import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreePath;

public class Main {
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new BorderLayout());
    frame.add(new TestPane());
    frame.pack();
    frame.setVisible(true);
  }
}
class TestPane extends JPanel {
  DefaultTreeModel model;
  JTree tree;
  public TestPane() {
    setLayout(new BorderLayout());
    tree = new JTree();
    File rootFile = new File(".");
    DefaultMutableTreeNode root = new DefaultMutableTreeNode(rootFile);
    model = new DefaultTreeModel(root);

    tree.setModel(model);
    tree.setRootVisible(true);
    tree.setShowsRootHandles(true);

    add(new JScrollPane(tree));

    JButton load = new JButton("Load");
    add(load, BorderLayout.SOUTH);

    load.addActionListener(e -> {
      DefaultMutableTreeNode r = (DefaultMutableTreeNode) model.getRoot();
      root.removeAllChildren();
      model.reload();
      File f = (File) r.getUserObject();
      addFiles(f, model, r);
      tree.expandPath(new TreePath(r));
    });
  }
  @Override
  public Dimension getPreferredSize() {
    return new Dimension(200, 200);
  }
  protected void addFiles(File rootFile, DefaultTreeModel model,
      DefaultMutableTreeNode root) {
    for (File file : rootFile.listFiles()) {
      DefaultMutableTreeNode child = new DefaultMutableTreeNode(file);
      model.insertNodeInto(child, root, root.getChildCount());
      if (file.isDirectory()) {
        addFiles(file, model, child);
      }
    }
  }
}