如何使用循环JFrame解决按钮求和

Question

如何使动作侦听器求和上述循环？我究竟做错了什么？也许我应该将总和放在第一个循环中，而不是另一个放到动作侦听器中。还是我的动作监听器有问题？请有人指出我的错误。

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

public class Bton extends javax.swing.JFrame {
JPanel pan = (JPanel) this.getContentPane();
JLabel []lab = new JLabel[4];
JTextField []text = new JTextField[4];
Border border = BorderFactory.createLineBorder(Color.grey,1);
JButton b = new JButton("Calculate");

public Bton() {
    initComponents();
    pan.setLayout(null);

    for(int i=0; i<lab.length ;i++){
        lab[i] = new JLabel();
        text[i] = new JTextField();

        lab[i].setBounds(new Rectangle(15,(i+1)*40, 60, 25));
        lab[i].setText("Box #" + (i+1));
        lab[i].setBorder(border);
        text[i].setBounds(new Rectangle(100,(i+1)*40, 60, 25));

        pan.add(lab[i],null);
        pan.add(text[i],null);

        setSize(200,420);
        setTitle("Sum tab.");
    }

    JLabel labe = new JLabel();
    labe.setBounds(new Rectangle(15, 330, 60, 25));
    labe.setText("Total Sum");
    labe.setBorder(border);
    JTextField txt = new JTextField(6);
    txt.setBounds(new Rectangle(100, 330, 60, 25));

    b.setBounds(70, 290, 89, 23);
    add(b);
    b.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            int sum = 0;
            for(int i=0; i<lab.length ;i++){
                sum =+ Integer.parseInt(text[i].getText());
            }
            txt.setText(String.valueOf(sum));                
        }
    });        
    pan.add(labe);
    pan.add(txt);
}