代码最后用lambda表达式写的方法出现了问题，帮忙搞定一下吧（自己写的计算器的代码）

Question

import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.border.LineBorder;

interface Calculable {
    double CalculateInt(double x, double y);
}// 用来定义Lambda表达式的接口

public class Mycalc extends JFrame {
    private static final long serialVersionUID = 1L;
    // 用来保存第一个数
    private String number1 = "";
    // 用来保存第二个数
    private String number2 = "";
    // 用来保存两个数的运算符
    private String operator = "";
    // 显示框
    JTextField jt = new JTextField("0.0 ");

    public Mycalc() {
        setTitle("My first Calc");
        setSize(600, 750);
        setLayout(null);
        SetCenter(this);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        addJT();
        addButton();
        setVisible(true);
    }

    void addJT() {// 添加显示框
        jt.setBounds(0, 0, 600, 150);
        // 不允许对显示框进行编辑
        jt.setEditable(false);
        // 显示框内的数据右对齐
        jt.setHorizontalAlignment(JTextField.RIGHT);
        // 设置背景色
        jt.setBackground(new Color(100, 100, 100));
        // 设置字体属性
        jt.setFont(new Font("宋体", Font.BOLD, 20));
        add(jt);// 添加
    }

    void addButton() {// 添加操作按钮
        JButton jb_7 = new JButton("7");
        // 设置按钮边框的属性
        jb_7.setBorder(new LineBorder(Color.BLUE, 1));
        // 设置按钮的字体属性
        jb_7.setFont(new Font("宋体", Font.BOLD, 20));
        // 设置按钮的位置及大小
        jb_7.setBounds(0, 150, 150, 150);
        // 添加数字监听者
        jb_7.addActionListener(new NumberBtListener());
        add(jb_7);// 添加

        JButton jb_8 = new JButton("8");
        jb_8.setBorder(new LineBorder(Color.BLUE, 1));
        jb_8.setFont(new Font("宋体", Font.BOLD, 20));
        jb_8.setBounds(150, 150, 150, 150);
        jb_8.addActionListener(new NumberBtListener());
        add(jb_8);

        JButton jb_9 = new JButton("9");
        jb_9.setBorder(new LineBorder(Color.BLUE, 1));
        jb_9.setFont(new Font("宋体", Font.BOLD, 20));
        jb_9.setBounds(300, 150, 150, 150);
        jb_9.addActionListener(new NumberBtListener());
        add(jb_9);

        JButton jb_add = new JButton("+");
        jb_add.setBounds(450, 150, 150, 150);
        jb_add.setBorder(new LineBorder(Color.BLUE, 1));
        jb_add.setFont(new Font("宋体", Font.BOLD, 20));
        jb_add.addActionListener(new Operation());
        add(jb_add);

        JButton jb_4 = new JButton("4");
        jb_4.setBorder(new LineBorder(Color.BLUE, 1));
        jb_4.setFont(new Font("宋体", Font.BOLD, 20));
        jb_4.setBounds(0, 300, 150, 150);
        jb_4.addActionListener(new NumberBtListener());
        add(jb_4);

        JButton jb_5 = new JButton("5");
        jb_5.setBorder(new LineBorder(Color.BLUE, 1));
        jb_5.setFont(new Font("宋体", Font.BOLD, 20));
        jb_5.setBounds(150, 300, 150, 150);
        jb_5.addActionListener(new NumberBtListener());
        add(jb_5);

        JButton jb_6 = new JButton("6");
        jb_6.setBorder(new LineBorder(Color.BLUE, 1));
        jb_6.setFont(new Font("宋体", Font.BOLD, 20));
        jb_6.setBounds(300, 300, 150, 150);
        jb_6.addActionListener(new NumberBtListener());
        add(jb_6);

        JButton jb_cut = new JButton("-");
        jb_cut.setBounds(450, 300, 150, 150);
        jb_cut.setBorder(new LineBorder(Color.BLUE, 1));
        jb_cut.setFont(new Font("宋体", Font.BOLD, 20));
        jb_cut.addActionListener(new Operation());
        add(jb_cut);

        JButton jb_1 = new JButton("1");
        jb_1.setBorder(new LineBorder(Color.BLUE, 1));
        jb_1.setFont(new Font("宋体", Font.BOLD, 20));
        jb_1.setBounds(0, 450, 150, 150);
        jb_1.addActionListener(new NumberBtListener());
        add(jb_1);

        JButton jb_2 = new JButton("2");
        jb_2.setBorder(new LineBorder(Color.BLUE, 1));
        jb_2.setFont(new Font("宋体", Font.BOLD, 20));
        jb_2.setBounds(150, 450, 150, 150);
        jb_2.addActionListener(new NumberBtListener());
        add(jb_2);

        JButton jb_3 = new JButton("3");
        jb_3.setBorder(new LineBorder(Color.BLUE, 1));
        jb_3.setFont(new Font("宋体", Font.BOLD, 20));
        jb_3.setBounds(300, 450, 150, 150);
        jb_3.addActionListener(new NumberBtListener());
        add(jb_3);

        JButton jb_ride = new JButton("*");
        jb_ride.setBounds(450, 450, 150, 150);
        jb_ride.setBorder(new LineBorder(Color.BLUE, 1));
        jb_ride.setFont(new Font("宋体", Font.BOLD, 20));
        jb_ride.addActionListener(new Operation());
        add(jb_ride);

        JButton jb_0 = new JButton("0");
        jb_0.setBorder(new LineBorder(Color.BLUE, 1));
        jb_0.setFont(new Font("宋体", Font.BOLD, 20));
        jb_0.setBounds(0, 600, 150, 150);
        jb_0.addActionListener(new NumberBtListener());
        add(jb_0);

        JButton jb_equal = new JButton("=");
        jb_equal.setBounds(150, 600, 150, 150);
        jb_equal.setBorder(new LineBorder(Color.BLUE, 1));
        jb_equal.setFont(new Font("宋体", Font.BOLD, 20));
        jb_equal.addActionListener(new ClearOrEqual());
        add(jb_equal);

        JButton jb_C = new JButton("C");
        jb_C.setBounds(300, 600, 150, 150);
        jb_C.setBorder(new LineBorder(Color.BLUE, 1));
        jb_C.setFont(new Font("宋体", Font.BOLD, 20));
        jb_C.addActionListener(new ClearOrEqual());
        add(jb_C);

        JButton jb_divide = new JButton("/");
        jb_divide.setBounds(450, 600, 150, 150);
        jb_divide.setBorder(new LineBorder(Color.BLUE, 1));
        jb_divide.setFont(new Font("宋体", Font.BOLD, 20));
        jb_divide.addActionListener(new Operation());
        add(jb_divide);
    }

    public static void main(String[] args) {
        new Mycalc();
    }

    // 让主窗口最初在屏幕的中间显示
    void SetCenter(Component cp) {
        // 得到屏幕的长度和宽度
        Dimension dim = 
                Toolkit.getDefaultToolkit().getScreenSize();
        int screenheight = dim.height;
        int screenwidth = dim.width;
        int x = (screenwidth - cp.getWidth()) / 2;
        int y = (screenheight - cp.getHeight()) / 2;
        cp.setLocation(x, y);
    }

    /**对数字按钮进行事件监听
     * 当未对运算符进行设置时，进行对第一个数的数值进行输入 
     * 运算符不为空时，对第二个数的数据进行输入
     */
    class NumberBtListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JButton jbt = (JButton) e.getSource();
            String content = jbt.getText();
            if (!operator.equals("")) {
                number1 += content;
                int number = Integer.valueOf(number1);
                jt.setText(number + " ");
            } else {
                number2 += content;
                int number = Integer.valueOf(number2);
                jt.setText(number + " ");
            }
        }
    }

    // 得出运算结果和清空显示框的监听者
    class ClearOrEqual implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JButton jbt = (JButton) e.getSource();
            String content = jbt.getText();
            // number用来存放两个数运算的结果
            double number = 0;
            // num1用来得到第一个数的数值
            double num1 = Double.parseDouble(number1);
            // num2用来得到第二个数的数值
            double num2 = Double.parseDouble(number2);
            // 得到此时运算的方法
            Calculable cal = Calculate(operator);
            switch (content) {
            case "=": {
                if (!(operator.equals("") && number2.equals(""))) {
                    // 运算完成
                    number = cal.CalculateInt(num1, num2);
                    // 将运算完的数值再送给number1
                    //在多次点击‘=’时，可以进行多次运算
                    number1 = String.valueOf(number);
                    jt.setText(number + " ");
                }
            }
                break;
            case "C": {
                jt.setText("0");
                number1 = "";
                number2 = "";
                operator = "";
            }
                break;
            }
        }
    }

    // 对运算符按钮进行监听的监听者
    class Operation implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JButton jbt = (JButton) e.getSource();
            String content = jbt.getText();
            // 将按钮的运算符送给operator
            operator = content;
            // 将number2置空
            number2 = "";
        }
    }

    // 利用lambda表达式求运算方法
    public static Calculable Calculate(String str) {
        Calculable result;
        switch (str) {
        case "+": {
            result = (x, y) -> (x + y);
        }
            break;
        case "-": {
            result = (x, y) -> (x - y);
        }
            break;
        case "*": {
            result = (x, y) -> (x * y);
        }
            break;
        case "/": {
            result = (x, y) -> (x / y);
        }
            break;
        }
        return result;
    }
}

Answer 1

首先，这段代码有一个编译错误，public static Calculable Calculate(String str) 这个方法没有对变量初始化无法编译通过。
其次，你的运算符号按钮响应有问题，=的响应报错。调试发现 double num2 = Double.parseDouble(number2); 这里的 number2 并没有收集到内容，而是空串。
根源在于你将 number2 的内容置空了：

class Operation implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JButton jbt = (JButton) e.getSource();
            String content = jbt.getText();
            // 将按钮的运算符送给operator
            operator = content;
            // 将number2置空
//          number2 = "";
        }
    }

去掉number2=""，基本功能能够运行了。