import javax.swing.JFrame;import javax.swing.JTree;import javax.swing.tree.DefaultMutableTreeNode;public class JTreeDemo extends JFrame{        /**     *      */    private static final long serialVersionUID = 1L;        public JTreeDemo(){        //根节点        DefaultMutableTreeNode group =new DefaultMutableTreeNode("根节点");        //二级节点        DefaultMutableTreeNode first=new DefaultMutableTreeNode("二级节点1");        DefaultMutableTreeNode second = new DefaultMutableTreeNode("二级节点2");        DefaultMutableTreeNode thrid = new DefaultMutableTreeNode("二级节点3");        //三级节点        DefaultMutableTreeNode first1 = new DefaultMutableTreeNode("三级节点11");        DefaultMutableTreeNode first2 = new DefaultMutableTreeNode("三级节点12");                DefaultMutableTreeNode second1 = new DefaultMutableTreeNode("三级节点21");        DefaultMutableTreeNode second2 = new DefaultMutableTreeNode("三级节点22");        DefaultMutableTreeNode second3 = new DefaultMutableTreeNode("三级节点23");                DefaultMutableTreeNode thrid1 = new DefaultMutableTreeNode("三级节点31");        DefaultMutableTreeNode thrid2 = new DefaultMutableTreeNode("三级节点32");        DefaultMutableTreeNode thrid3 = new DefaultMutableTreeNode("三级节点33");                //将三级节点添加到二级节点上        first.add(first1);        first.add(first2);                second.add(second1);        second.add(second2);        second.add(second3);                thrid.add(thrid1);        thrid.add(thrid2);        thrid.add(thrid3);                //将二级节点添加到一级节点上        group.add(first);        group.add(second);        group.add(thrid);                JTree tree = new JTree(group);        //以group为参数创建根目录        tree.collapseRow(1);                //总是显示根目录在前        tree.setToggleClickCount(1);        //设置鼠标单击数                this.getContentPane().add(tree);        setSize(300,300);        setVisible(true);        setTitle("树的应用");    }        public static void main(String[] args) {        new JTreeDemo();    }}