帮我改改这个“能用加法的Java计算器”

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator extends JPanel
{
static JFrame frame;
JLabel add;
JTextField num1TextField,num2TextField,resultTextField;
JButton equalButton;

public Calculator()
{
num1TextField = new JTextField(5);
num2TextField = new JTextField(5);
resultTextField = new JTextField(5);
add = new JLabel("+");
equalButton = new JButton("=");
add(num1TextField);
add(num2TextField);
add(add);
add(equalButton);
add(resultTextField);

ButtonListener bnListener = new ButtonListener();
equalButton.addActionListener(bnListener);
}
class ButtonListener implements ActionListener{
public void actionPerformed(ActionEvent e){
Double n1 = new Double(num1TextField.getText());
Double n2 = new Double(num2TextField.getText());