Java GUI计算器：值不是恒定的？

Question

我在Java上制作GUI计算器。我已经放入了actionlistener和按钮和东西，按钮正在被听到，但每当我按两次数字时，都不会出现两位数;只会显示第二个数字。在我的if语句中，如果没有选择一个操作（如+或 - ），那么按下的任何数字都将是第一个数字的一​​部分，比如按1两次会产生11，而不是1和1分开 - 但这就是我的计算器无论如何。如果选择了一个操作，那么它将转到第二个数字。我还没有开始我的同等按钮，但我希​​望首先正确形成数字。我把我的代码减少到两个数字，所以它在这里更短。感谢任何帮助过的人！

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.util.Random;
import java.awt.FlowLayout;

public class CalculatorY {
    public static  JPanel panel = new JPanel(new GridLayout(5, 5));
    public static JButton one, two, add;
    public static JTextField result;

    public static void main(String[] args) {
        JFrame frame = new JFrame("calc");
        result = new JTextField(null, 8);
        one = new JButton("1");
        two = new JButton("2");
        add = new JButton("+");
        add.addActionListener(new button());
        one.addActionListener(new button());
        two.addActionListener(new button());
        panel.setLayout(new GridLayout(5, 5, 5, 25));
        panel.setLayout(new FlowLayout());
        panel.add(result);
        panel.add(one);
        panel.add(two);
        panel.add(add);
        frame.setVisible(true);
        frame.setSize(200, 400);
        frame.setResizable(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setContentPane(panel);
    }

    public static class button implements ActionListener {
        public void actionPerformed(ActionEvent event) {

            String command = event.getActionCommand();
            String text = result.getText();
            int textLength = text.length();
            String letterValueOne = " ";
            String letterValueTwo = " ";
            String op = " ";

            String valueOne;
            switch (command) {
                case "1":
                        if (op == " ") {
                        if (letterValueOne == " ") {
                            letterValueOne = "1";   
                            System.out.println(letterValueOne);

                        } else {
                            letterValueOne = letterValueOne + 1;

                        }
                    } else {
                        if (letterValueOne != " " && letterValueTwo == " ") {
                            letterValueTwo = "1";
                        } else {
                            letterValueTwo = letterValueTwo + 1;     
                        }
                    }
                    break;
                case "2":
                    if (op == " ") {
                        if (letterValueOne == " ") {
                            letterValueOne = "2";   
                            System.out.println(letterValueOne);  
                        } else {
                            letterValueOne = letterValueOne + 2;

                        }
                    } else {
                        if (letterValueOne != " " && letterValueTwo == " ") {
                            letterValueTwo = "2";
                        } else {
                            letterValueTwo = letterValueTwo + 2;

                        }
                    }
                    break;
                case "+":
                    System.out.println("+");
                    if (result == null) {
                        result = null;
                    } else if (letterValueOne != "") {
                        op = "+";
                        result.setText(letterValueOne + op);
                    } else if (letterValueOne != " " && letterValueTwo != " ") {
                        result.setText(letterValueOne + op + letterValueTwo);
                        op = "+";
                    }
                    break;
                }
                result.setText(letterValueOne + op + letterValueTwo);
        }
    }
}