大家好，我用JAVA语言事件处理机制内部类时的问题，希望大家帮忙

package b1;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
//import java.util.*;
public class GuiAdd
{ static JTextField txt1;//为什么要定义为静态变量？？？
static JTextField txt2;
static JTextField txt3;
public static void main(String[] arps)
{
txt1=new JTextField(10);
txt2=new JTextField(10);
txt3=new JTextField(10);
JFrame jframe=new JFrame("求和");
jframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
jframe.setSize(400,300);
JPanel rootpanel=(JPanel)jframe.getContentPane();
rootpanel.setLayout(null);
JLabel label1=new JLabel("数字1");
label1.setFont(new Font("宋体",Font.PLAIN,13));
label1.setBounds(40, 30, 40, 15);
JLabel label2=new JLabel("数字2");
label2.setFont(new Font("宋体",Font.PLAIN,13));
label2.setBounds(200, 30, 40, 15);
JPanel j1=new JPanel();
j1.setSize(400,100);
j1.setBackground(Color.cyan);
j1.add(label1);
j1.add(txt1);
j1.add(label2);
j1.add(txt2);
JButton but=new JButton("求和");
JPanel j2=new JPanel();
j2.setSize(400,70);
j2.setBackground(Color.pink);
j2.setLocation(0,100);
j2.add(but);
but.addActionListener(new add(this));//添加按钮监听器，为什么出错？？？？
JPanel j3=new JPanel();
j3.setSize(400,130);
j3.setBackground(Color.green);
j3.setLocation(0,170);
j3.add(txt3);
rootpanel.add(j1);
rootpanel.add(j2);
rootpanel.add(j3);
jframe.setVisible(true);
jframe.getRootPane().setDefaultButton(but);
}
class add extends ActionListener //定义内部类为什么不对？？？
{
public void actionPerformed(ActionEvent event)
{
double a = Double.parseDouble(txt1.getText());
double b = Double.parseDouble(txt2.getText());
double c = a+b;
//String s=String.valueOf(c);
txt3.setText(c+"");
}
}
}