树控件通过setEditable(true)方法设置可编辑条件下，可以添加新节点了。为当前选中的节点添加新的节点有两点需要考虑（主要考虑节点保存的用户对象是字符串的情况）：
1.是否存在相同的节点（查找保存的用户对象是否相同）代码如下：

    private boolean isfindNode(DefaultMutableTreeNode dmtn, String text)
    {
        if(dmtn.getChildCount()==0)
            return false;
        int count = dmtn.getChildCount();
        for(int i=0;i<count;i++)
        {
            DefaultMutableTreeNode t = (DefaultMutableTreeNode) dmtn.getChildAt(i);
            String str = (String) t.getUserObject();
            if(text.equals(str))
                return true;
        }
        return false;
    }

2.添加完节点，让新节点处于被选择状态。代码如下：

//转到新添加的节点
    private void scrollToNode(DefaultMutableTreeNode t)
    {
        DefaultTreeModel dtm = (DefaultTreeModel) tree.getModel();
        //TreeNode[] tns = dtm.getPathToRoot(t);
        TreeNode[] tns = t.getPath();
        TreePath tp = new TreePath(tns);
        tree.scrollPathToVisible(tp);
        tree.setSelectionPath(tp);//当前选择的是tns的父节点，需要tns被选择
        tree.updateUI();
    }

全部代码如下：

package tree;

import java.awt.BorderLayout;
import java.awt.Image;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.AbstractButton;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JTree;
import javax.swing.border.TitledBorder;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeCellRenderer;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreeNode;
import javax.swing.tree.TreePath;


public class CopeTree extends JFrame
{
    public  JTree tree;
    static int index = 0;
    JTextField jtf ;
    public CopeTree()
    {
        JPanel panel = new JPanel();
        initTree();
        add(tree);
        tree.setBorder(new TitledBorder("普通树控件"));
        jtf = new JTextField(30);
        jtf.addActionListener(event->{
            addNode();
        });
        panel.add(jtf);
        JButton jbn_add = new JButton("添加节点");
        jbn_add.addActionListener(event -> {
            addNode();
            }
        );
        panel.add(jbn_add);
        add(panel, BorderLayout.SOUTH);
        init();
    }
    private void addNode()
    {
        
        String text = jtf.getText();
        if (text == null || text.equals("") || text.trim().equals(""))
        {
            JOptionPane.showMessageDialog(this, "节点名称不能为空");
            return;
        }
        DefaultMutableTreeNode tmtn = (DefaultMutableTreeNode) tree.getLastSelectedPathComponent();
        if (tmtn != null)
        {
            boolean result = isfindNode(tmtn, text);
            if (result)
            {
                JOptionPane.showMessageDialog(this, "存在相同节点"+text);
                jtf.requestFocus();
                jtf.selectAll();
                return;// 存在相同节点
            }
            DefaultMutableTreeNode t = new DefaultMutableTreeNode(text);
            tmtn.add(t);
            jtf.setText("");
            scrollToNode(t);
        }
        else
        {
            JOptionPane.showMessageDialog(this, "没有选择节点");
            return;
        }

    }
    //转到新添加的节点
    private void scrollToNode(DefaultMutableTreeNode t)
    {
        DefaultTreeModel dtm = (DefaultTreeModel) tree.getModel();
        //TreeNode[] tns = dtm.getPathToRoot(t);
        TreeNode[] tns = t.getPath();
        TreePath tp = new TreePath(tns);
        tree.scrollPathToVisible(tp);
        tree.setSelectionPath(tp);//当前选择的是tns的父节点，需要tns被选择
        tree.updateUI();
    }

    private boolean isfindNode(DefaultMutableTreeNode dmtn, String text)
    {
        if(dmtn.getChildCount()==0)
            return false;
        int count = dmtn.getChildCount();
        for(int i=0;i<count;i++)
        {
            DefaultMutableTreeNode t = (DefaultMutableTreeNode) dmtn.getChildAt(i);
            String str = (String) t.getUserObject();
            if(text.equals(str))
                return true;
        }
        return false;
    }

    private void initTree()
    {
        // TODO Auto-generated method stub
        DefaultMutableTreeNode root = new DefaultMutableTreeNode("科学");
        DefaultMutableTreeNode shuxue = new DefaultMutableTreeNode("数学");
        DefaultMutableTreeNode jisuanji = new DefaultMutableTreeNode("计算机");
        root.add(shuxue);
        root.add(jisuanji);
        addTreeNode(shuxue,new String[] {"数论","几何","代数"});
        addTreeNode(jisuanji,new String[] {"编程语言","操作系统","算法"});
        tree = new JTree(root);
        tree.setEditable(true);
    }

    private void addTreeNode(DefaultMutableTreeNode parent, String[] strs)
    {
        for(int i = 0;i<strs.length;i++)
        {
            DefaultMutableTreeNode temp = new DefaultMutableTreeNode(strs[i]);
            parent.add(temp);
        }
        
    }
    public void init()
    {

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(600, 500);
        this.setLocationRelativeTo(null);
        this.setTitle("树控件");
        this.setVisible(true);
    }
}

运行如下：

1.png

2.png

由于水平有限，如果有错误，请大家多多指导，提高水平，共同学习。