jtree节点问题

Question

DefaultMutableTreeNode page[]=new DefaultMutableTreeNode[200];
我首先定义一个节点数组，由于节点显示的值从数据库中调，
page[i]=new DefaultMutableTreeNode(rs3.getString("pageno"));
root.add(page[i]);
这样可以实现，我得问题是我的page[i]=new 的new是不是重复定义了page[]这个数组？
其实我只想把rs3.getString("pageno"))赋给page[i]节点，我曾尝试
Object pag=rs3.getString("pageno");
  page[i].setUserObject(pag);可是运行出错，
Exception in thread "main" java.lung.NullPointerException,怎么回事，如何解决？

Answer 1

DefaultMutableTreeNode page[]=new DefaultMutableTreeNode[200];
我首先定义一个节点数组，由于节点显示的值从数据库中调，
page[i]=new DefaultMutableTreeNode(rs3.getString("pageno"));
root.add(page[i]);
这个并没有重复定义呀。。
DefaultMutableTreeNode page[]=new DefaultMutableTreeNode[200];
声明了你的数组里放的什么类型。
page[i]=new DefaultMutableTreeNode(rs3.getString("pageno"));
对树组里面的元素实例化。。并没有重复定义

Answer 2

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
import com.incors.plaf.alloy.*;//组件的下载网址http://www.incors.com/lookandfeel/
/*将alloy.jar放在c:\j2sdk1.4.0\jre\lib\ext\目录下.
 */
public class TreeDemo3
{
    public TreeDemo3()
    {
        JFrame f = new JFrame("TreeDemo");
        Container contentPane = f.getContentPane();
        
        
        DefaultMutableTreeNode root = new DefaultMutableTreeNode("资源管理器");
        DefaultMutableTreeNode node1 = new DefaultMutableTreeNode("我的公文包");
        DefaultMutableTreeNode node2 = new DefaultMutableTreeNode("我的电脑");
        DefaultMutableTreeNode node3 = new DefaultMutableTreeNode("收藏夹");
        DefaultMutableTreeNode node4 = new DefaultMutableTreeNode("Readme");
        
        DefaultTreeModel treeModel = new DefaultTreeModel(root);        /*DefaultTreeModel类所提供的insertNodeInto()方法加入节点到父节点的数量.
         *利用DefaultMutableTreeNode类所提供的getChildCount()方法取得目前子节点的数量.
         */
        treeModel.insertNodeInto(node1, root, root.getChildCount());
        treeModel.insertNodeInto(node2, root, root.getChildCount());
        treeModel.insertNodeInto(node3, root, root.getChildCount());
        treeModel.insertNodeInto(node4, root, root.getChildCount());
        
        DefaultMutableTreeNode leafnode = new 
                DefaultMutableTreeNode("公司文件");        //DefaultTreeModel类所提供的insertNodeInto()方法加入节点到父节点的数量.
        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());
        leafnode = new DefaultMutableTreeNode("个人信件");
        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());
        leafnode = new DefaultMutableTreeNode("私人文件");
        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());
        
        leafnode = new DefaultMutableTreeNode("本机磁盘(C:)");
        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());
        leafnode = new DefaultMutableTreeNode("本机磁盘(D:)");
        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());
        leafnode = new DefaultMutableTreeNode("本机磁盘(E:)");
        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());
        
        DefaultMutableTreeNode node31 = new DefaultMutableTreeNode("网站列表");
        treeModel.insertNodeInto(node31, node3, node3.getChildCount());
        leafnode = new DefaultMutableTreeNode("奇摩站");
        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());
        leafnode = new DefaultMutableTreeNode("职棒消息");
        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());
        leafnode = new DefaultMutableTreeNode("网络书店");
        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());
        try {
           LookAndFeel alloyLnF = new AlloyLookAndFeel();    
           UIManager.setLookAndFeel(alloyLnF);
        } catch (UnsupportedLookAndFeelException ex) {
        // You may handle the exception here
        }
         // this line needs to be implemented in order to make JWS work properly
          UIManager.getLookAndFeelDefaults().put("ClassLoader", getClass().getClassLoader());
        
        //以TreeModel建立JTree。
        JTree tree = new JTree(treeModel);
        /*改变JTree的外观**/
          tree.putClientProperty("JTree.lineStyle","Horizontal");
        /*改变JTree的外观**/
        JScrollPane scrollPane = new JScrollPane();
        scrollPane.setViewportView(tree);
        
        contentPane.add(scrollPane);
        f.pack();
        f.setVisible(true);
        
        f.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });    }    public static void main(String args[]) {
    
        new TreeDemo3();
    }
}

Answer 3

1、DefaultMutableTreeNode page[]=new DefaultMutableTreeNode[200];
只定义了一个length为200的DefaultMutableTreeNode数组，只表明这个拥有200个元素的数组
它的每个元素都指向DefaultMutableTreeNode类型。
2、page[i]=new DefaultMutableTreeNode(rs3.getString("pageno"));
只有这个时候才真正创建了对象所以只想把rs3.getString("pageno"))赋给page[i]节点
Object pag=rs3.getString("pageno");
  page[i].setUserObject(pag);肯定报NullPointerException错