我是java新手，自学java。用java写一个JFrame的窗口，能够读取本地电脑上的文件。设置一个JTextArea文本域，用于显示读取的文件的内容。窗口下方有三个按钮（JButton），分别为“打开”、“保存”、“另存为”。打开窗口时，默认“打开”按钮可以点击，其他两个按钮不能点击。当读取文件到JTextArea文本区后，“保存”按钮激活，可以点击。我用下面的代码实现。其中DocumentListener我用两种方式实现：一种是直接在 JFrameTransFile类中实现，在该类内部调用，这种方式是可以修改JButton按钮是否激活属性的；另一种是另外写一个类Swing_OnValueChanged实现DocumentListener接口，在JFrameTransFile类中设置文本区（JTextArea）监听（代码中** **部分），调用Swing_OnValueChanged，此时会报错（报错信息附在代码后面），是在Swing_OnValueChanged类中insertUpdate()方法中修改JFrameTransFile类中的jbutton2的属性是报错的。请高手指教，如果我想在文本区变化时，调用Swing_OnValueChanged类修改JFrameTransFile类中JButton的setEnable属性，该怎么修改代码？在此先谢谢各位大侠了。
以下为部分主要代码：
package demo.java.transfile;
import java.awt.*;
import java.io.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.Document;
public class JFrameTransFile extends WindowAdapter implements ActionListener,TextListener,DocumentListener
{
JFrame jf;//框架变量
JTextArea jtextarea1;//多行文本区
JPanel jp1;
JTextField jtextfield1;//单行文本框
JScrollPane jscrollpane1;
JButton jbutton1,jbutton2,jbutton3;//按钮
FileDialog filedialog1;//文件对话框
File file1=null;//文件对象初始化为空值
public void display()
{//设置界面与监听
jf=new JFrame("坐标转换");
jf.setSize(680,400);
jf.addWindowListener(this);
.........
.........
//定义监听
DocumentListener sovc=new Swing_OnValueChanged();
//设置滚动条
jscrollpane1=new JScrollPane();
jf.add(jscrollpane1);
//添加文本区
jtextarea1=new JTextArea();
jtextarea1.setEnabled(false);
jscrollpane1.setViewportView(jtextarea1);
**//添加文本区监听（调用另外的类中实现的DocumentListener）**
**jtextarea1.getDocument().addDocumentListener(sovc);**
**//添加文本区监听（调用另外的类中实现的DocumentListener）**
**//jtextarea1.getDocument().addDocumentListener(this);**
//面板与布局
jp1=new JPanel();
jp1.setLayout(new FlowLayout(FlowLayout.LEFT));
jbutton1=new JButton("打开");
jbutton2=new JButton("保存");
jbutton3=new JButton("另存为");
jp1.add(jbutton1);
jp1.add(jbutton2);
jp1.add(jbutton3);
jbutton2.setEnabled(false);
jbutton3.setEnabled(false);
jbutton1.addActionListener(this);
jbutton2.addActionListener(this);
jbutton3.addActionListener(this);
jf.add(jp1,"South");
jf.setVisible(true);
}
public void textValueChanged(TextEvent e)
{}
public void actionPerformed(ActionEvent e)
{
if(e.getSource()==jbutton1)
{
//创建并打开文件对话框
filedialog1=new FileDialog(jf,"打开",FileDialog.LOAD);
filedialog1.setVisible(true);
if((filedialog1.getDirectory()!=null)&&(filedialog1.getFile()!=null))
{
jtextfield1.setText(filedialog1.getDirectory()+filedialog1.getFile());
try
{
//为文件对象赋值
file1=new File(filedialog1.getDirectory(),filedialog1.getFile());
//文件读入通道连向文件
FileReader fr=new FileReader(file1);
//定义文件缓冲区
BufferedReader br=new BufferedReader(fr);
String line;
//按行读取文本，每行附加在多行文本区之后
while((line=br.readLine())!=null)
{
jtextarea1.append(line.trim()+"\r\n");
}
fr.close();
br.close();
}
catch(IOException ioe)
{
System.out.println(ioe);
}
}
}
}
public void changedUpdate(DocumentEvent e)
{}
public void insertUpdate(DocumentEvent e)
{
jbutton2.setEnabled(true);
}
public void removeUpdate(DocumentEvent e)
{
}
public static void main(String[] args)
{
new JFrameTransFile().display();
}
}
class Swing_OnValueChanged implements DocumentListener
{
JFrameTransFile jftf=new JFrameTransFile();
public void changedUpdate(DocumentEvent e)
{}
public void insertUpdate(DocumentEvent e)
{
jftf.jbutton2.setEnabled(true);
}
public void removeUpdate(DocumentEvent e)
{}
}
以下部分为点击“打开”按钮时的报错
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at demo.java.transfile.Swing_OnValueChanged.insertUpdate(JFrameTransFile.java:150)
at javax.swing.text.AbstractDocument.fireInsertUpdate(Unknown Source)
at javax.swing.text.AbstractDocument.handleInsertString(Unknown Source)
at javax.swing.text.AbstractDocument.insertString(Unknown Source)
at javax.swing.text.PlainDocument.insertString(Unknown Source)
at javax.swing.JTextArea.append(Unknown Source)
at demo.java.transfile.JFrameTransFile.actionPerformed(JFrameTransFile.java:107)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
