70高分``在线等``Jtree问题！

package scar固定资产管理系统;import java.awt.*;import javax.swing.*;
import javax.swing.border.TitledBorder;
import javax.swing.tree.DefaultMutableTreeNode;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.tree.*;public class newclass extends JDialog {
    JPanel panel1 = new JPanel();
    JScrollPane jScrollPane1 = new JScrollPane();
    Box box1 = Box.createVerticalBox();
    TitledBorder titledBorder1 = new TitledBorder("");
    JTree tree;
    JButton jButton1 = new JButton();
    JButton jButton2 = new JButton();
    JButton jButton3 = new JButton();
    JButton jButton4 = new JButton();
    DefaultMutableTreeNode leaft;    public newclass(Frame owner, String title, boolean modal) {
        super(owner, title, modal);
        try {
            setDefaultCloseOperation(DISPOSE_ON_CLOSE);
            jbInit();
            pack();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
    DefaultMutableTreeNode root=new DefaultMutableTreeNode("全部资产");
    DefaultMutableTreeNode oot=new DefaultMutableTreeNode("ACCP软件教材");
    DefaultMutableTreeNode leif=new DefaultMutableTreeNode("网页编程");
    JButton jButton5 = new JButton();
    JButton jButton6 = new JButton();    public newclass() {
        this(new Frame(), "newclass", false);
    }    private void jbInit() throws Exception {
        panel1.setLayout(null);
        this.setResizable(false);
        this.setTitle("类别管理");        jButton1.addActionListener(new newclass_jButton1_actionAdapter(this));
        jButton2.addActionListener(new newclass_jButton2_actionAdapter(this));
        jButton5.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton5.setText("保    存");
        jButton6.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton6.setText("退    出");
        jButton3.addActionListener(new newclass_jButton3_actionAdapter(this));
        oot.add(leif);
        root.add(oot);
        tree=new JTree(root);        tree.setBounds(0,0,300,300);
        tree.setVisible(true);
        jScrollPane1.getViewport().setBackground(Color.white);
        jScrollPane1.setBorder(null);
        jScrollPane1.setBounds(new Rectangle( -3, 50, 300, 250));
        box1.setBackground(Color.lightGray);
        box1.setBounds(new Rectangle(313, 51, 85, 249));
        jButton1.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton1.setText("增加项目");
        jButton2.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton2.setText("增加子项");
        jButton3.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton3.setText("变更名称");
        jButton4.setFont(new java.awt.Font("新宋体", Font.PLAIN, 12));
        jButton4.setText("删除此类");
        getContentPane().add(panel1);
        panel1.add(box1);
        box1.add(jButton1);
        box1.add(jButton2);
        box1.add(jButton3);
        box1.add(jButton4);
        box1.add(jButton5);
        box1.add(jButton6);
        panel1.add(jScrollPane1);
        jScrollPane1.getViewport().add(tree);
    }    public void jButton1_actionPerformed(ActionEvent e) {
        if (tree.isSelectionEmpty())
        {
            javax.swing.JOptionPane.showMessageDialog(this,"你还没选中","提示",0);
        }
        else
        {
            leaft =new DefaultMutableTreeNode("新建类别");
            ((DefaultMutableTreeNode)tree.getLastSelectedPathComponent()).add(leaft);
            root.add(leaft);
            tree.updateUI();
        }    }    public void jButton2_actionPerformed(ActionEvent e) {
        if (tree.isSelectionEmpty())
        {
            javax.swing.JOptionPane.showMessageDialog(this,"你还没选中","提示",0);
        }
        else
        {    //请问新建子类如何被展开
            leaft = new DefaultMutableTreeNode("新建子类");
            ((DefaultMutableTreeNode) tree.getLastSelectedPathComponent()).add(leaft);
            tree.updateUI();        }
    }    public void jButton3_actionPerformed(ActionEvent e) {
        if (tree.isSelectionEmpty())
        {
            javax.swing.JOptionPane.showMessageDialog(this,"你还没选中","提示",0);
        }
        else
        {
            ((DefaultMutableTreeNode) tree.getLastSelectedPathComponent()).setUserObject("重副名");
        }
    }
}
class newclass_jButton3_actionAdapter implements ActionListener {
    private newclass adaptee;
    newclass_jButton3_actionAdapter(newclass adaptee) {
        this.adaptee = adaptee;
    }    public void actionPerformed(ActionEvent e) {
        adaptee.jButton3_actionPerformed(e);
    }
}
class newclass_jButton1_actionAdapter implements ActionListener {
    private newclass adaptee;
    newclass_jButton1_actionAdapter(newclass adaptee) {
        this.adaptee = adaptee;
    }    public void actionPerformed(ActionEvent e) {
        adaptee.jButton1_actionPerformed(e);
    }
}
class newclass_jButton2_actionAdapter implements ActionListener {
    private newclass adaptee;
    newclass_jButton2_actionAdapter(newclass adaptee) {
        this.adaptee = adaptee;
    }    public void actionPerformed(ActionEvent e) {
        adaptee.jButton2_actionPerformed(e);
    }
}
请问大家我做的这个``增加子类时 如何直接展开来？ 谢谢了。请大家帮我啊``