GUI对话框演示代码为什么会报出空指针异常 - 黑马程序员技术交流社区

import java.awt.*;
import java.awt.event.*;
import java.io.*;
class WindowDemo
{
private Frame f;
private Button but;
private TextField tf;
private TextArea ta;
private Dialog d;
private Label l;
private Button okBut;
WindowDemo()
{
init();
}
public void init()
{
//设置窗口基本信息
f=new Frame("我的电脑");
but=new Button("转到");
tf=new TextField(50);
ta=new TextArea(20,60);
f.setBounds(200,100,500,400);
//将组件添加到Frame中
f.add(tf);
f.add(but);
f.add(ta);
//设置布局
f.setLayout(new FlowLayout());
//显示窗体
f.setVisible(true);
event();
//设置对话框基本信息
d=new Dialog(f,"提示信息",true);
d.setBounds(240,150,200,150);
l=new Label();
okBut=new Button("确定");
//添加组件
d.add(l);
d.add(okBut);
//设置布局
d.setLayout(new FlowLayout());
}
private void event()
{
//窗口监听：退出
f.addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
System.exit(0);
}
});
d.addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
d.setVisible(false);
}
});
okBut.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
d.setVisible(false);
}
});
//按钮监听
but.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
String dirPath=tf.getText();
File dir=new File(dirPath);
if(dir.exists()&&dir.isDirectory())
{
ta.setText("");
String []names=dir.list();
for(String name:names)
{
ta.append(name+"\r\n");
}
}
else
{
String info="您输入的信息："+dirPath+"是错误的";
l.setText(info);
//显示对话框
d.setVisible(true);
}
}
});
}
public static void main(String[] args)
{
new WindowDemo();
}
}

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