java frame实现简易计算器的方法

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class CalcDoc {

    static ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName("JavaScript");

    public static void main(String[] args) {
        final JFrame jf = new JFrame("计算器");
// 流动布局，居中
        jf.setLayout(new FlowLayout(1, 5, 5));
        jf.setSize(293, 260);
//
        jf.setLocationRelativeTo(null);
        jf.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
// 不可以改变大小
        jf.setResizable(false);

// 创建容器,存放显示数字框和 C 按钮
        JPanel panel1 = new JPanel();
// 定义按钮面板，并设置为网格布局，4行4列，组件水平、垂直间距均为3
        JPanel panel2 = new JPanel(new GridLayout(4, 4, 1, 1));

        // 创建文本域，放置输入数据和结果
        JTextArea text = new JTextArea(1, 13);
// 设置字体
        text.setFont(new Font(null, Font.PLAIN, 20));
// 设置大小
        text.setPreferredSize(new Dimension(190, 28));
// 不可编辑
        text.setEditable(false);
        panel1.add(text);

// 创建一个清除按钮'C'
        JButton btnC = new JButton("C");
        btnC.setFont(new Font(null, Font.PLAIN, 20));
        btnC.setPreferredSize(new Dimension(55, 28));
        panel1.add(btnC);

        // C 键 功能
        btnC.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String textStr = text.getText();
                if (textStr.indexOf("=") != -1) {
                    text.setText("");
                } else {
                    textStr = textStr.substring(0, (textStr.length() - 1));
                    text.setText(textStr);
                }
            }

        });

// 按钮数组
        String btn_str[] = {"1", "2", "3", "/", "4", "5", "6", "*", "7", "8", "9", "-", "0", ".", "=", "+"};

        JButton btn[] = new JButton[btn_str.length];
        for (int i = 0; i < btn_str.length; i++) {
            btn[i] = new JButton(btn_str[i]);
            btn[i].setFont(new Font(null, Font.PLAIN, 20));
            btn[i].setPreferredSize(new Dimension(70, 42));
            panel2.add(btn[i]);
        }

        // 按钮功能
        for (int i = 0; i < btn_str.length; i++) {
            if (i != 14) {
                // 如果不是等于号
                int j = i;
                btn[i].addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        // 获取文本框内容
                        String textStr = text.getText();
                        // 如果有等号，清空文本框
                        if (textStr.indexOf("=") != -1) {
                            text.setText("");
                        }
                        // 获取按钮字符添加到文本框
                        String s = btn[j].getText();
                        text.append(s);
                    }

                });
            } else {
                // 如果点击等于号
                btn[i].addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        try {
                            // 获取文本框内容
                            String gongshi = text.getText();
                            // 计算公式
                            String jieguo = scriptEngine.eval(gongshi).toString();

                            // 如果结果字符串过长，只显示等号和结果
                            if (jieguo.length() > 18) {
                                jieguo = jieguo.substring(jieguo.length() - 16, jieguo.length());
                                text.setText("=");
                                text.append(jieguo);
                            } else {
                                text.append("=");
                                text.append(jieguo);
                            }
                            //如果出错
                        } catch (Exception t) {
                            text.setText("");
                        }
                    }
                });
            }
        }
        {
        }// 皮皮狗子写的！


// 把 面板容器 设置到 窗口
        // panel1 设置到顶部
        jf.getContentPane().add(panel1, BorderLayout.NORTH);
        // panel1 设置到底部
        jf.getContentPane().add(panel2, BorderLayout.CENTER);
// 显示窗口，前面创建的信息都在内存中，通过 jf.setVisible(true) 把内存中的窗口显示在屏幕上。
        jf.setVisible(true);
    }
}