编写一个能四则运算的计算机程序，哪位大佬讲解一下？

package com.huawei.test;

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.util.List;
import java.util.regex.*;

import javax.swing.*;

public class Cal extends JFrame implements ActionListener {
    /**
     *
     */
    private static final long serialVersionUID = 1L;
    JTextField jtf = new JTextField();
    public Cal() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        add(jtf, BorderLayout.NORTH);
        jtf.setEditable(false);
        JPanel jp = new JPanel();
        List<String> list = new ArrayList<String>(20);
        List<String> list2 = new ArrayList<String>(Arrays.asList("+", "C", "-", "退格",
                "*",  "1/x", "+/-", ".", "/", "="));
        for(int i = 1; i < 10; i++) {
            list.add(i+"");
            if(i % 3 == 0) {
                list.addAll(list2.subList(0, 2));
                list2.remove(0);
                list2.remove(0);
            }
        }
        list.add("0");
        list.addAll(list2);
        jp.setLayout(new GridLayout(4, 5));
        for(String str : list) {
            JButton btn = new JButton(str);
            btn.addActionListener(this);
            jp.add(btn);
        }
        add(jp, BorderLayout.CENTER);
        pack();
        setLocation(200, 400);
        setResizable(false);
    }
    static String preTxt = null;
    @Override
    public void actionPerformed(ActionEvent e) {
        String btnTxt = ((JButton)e.getSource()).getText();
        if("C".equals(btnTxt)) {
            jtf.setText("");
        } else if("1/x".equals(btnTxt)) {
            double num = 0;
            try {
                num = Double.parseDouble(jtf.getText());
            } catch (Exception e2) {
                System.out.println(e2.getMessage());
            }
            jtf.setText(1/num+"");
        } else if("退格".equals(btnTxt)) {
            String content = jtf.getText();
            if(content.length()>0) {
                content = content.substring(0, content.length()-1);
            }
            jtf.setText(content);
        } else if("+/-".equals(btnTxt)) {
            String content = jtf.getText();
            if(content.startsWith("-")) {
                content = content.substring(1, content.length());
            } else {
                content = "-" + content;
            }
            jtf.setText(content);
        } else if("=".equals(btnTxt)) {
            String content = jtf.getText();
            content = getByRegex(content);
            jtf.setText(content);
        } else if(!"=".equals(preTxt)){
            jtf.setText(jtf.getText()+btnTxt);
        } else {
            jtf.setText(btnTxt);
        }
        preTxt = btnTxt;
    }
    private static String calc(String content, String regex) {
        String pattern = regex;
        Pattern r = (pattern);
        Matcher m = r.matcher(content);
        if (m.find( )) {
            int a = Integer.parseInt(m.group(1));
            int b = Integer.parseInt(m.group(3));
            int res=0;
            if("*".equals(m.group(2))) {
                res = a*b;
            }else if("/".equals(m.group(2))) {
                res = a/b;
            }else if("+".equals(m.group(2))) {
                res = a+b;
            }else if("-".equals(m.group(2))) {
                res = a-b;
            }
            return content.replace(m.group(), res+"");
        }
        return content;
    }
    private String getByRegex(String content) {
        for(String type : Arrays.asList("*", "/", "+", "-")) {
            content = calc(content, "(\\d+)(\\"+type+")(\\d+)");
            String content2 = null;
            while(!content.equals(content2)) {
                content2 = content;
                content = calc(content, "(\\d+)(\\"+type+")(\\d+)");
            }
        }
        return content;
    }
    public static void main(String[] args) {
        new Cal();
    }
}