Java 编写的计算器为什么不能显示出结果,该怎么处理

import java.awt.*;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import javax.swing.JFrame;import javax.swing.JOptionPane;import javax.swing.JPanel;import javax.swing.JTextField;public class Calculator extends Frame {        private double value = 0.0;    private String fuhao = "";    private String valueString = "0";    private int flag = 0;        Button[] b = new Button[18];    JPanel jp = new JPanel();    JTextField tf = new JTextField("0", 20);        public static void main(String[] args) {        Calculator cal = new Calculator();        cal.launchframe();    }    public void launchframe() {        this.setTitle("计算器 V1.0");        this.setVisible(true);        this.setResizable(false);        this.setBounds(450, 150, 230, 320);        jp.setLayout(new GridLayout(5, 4));        this.add(jp);        Draw();        MenuBar mb = new MenuBar();        this.setMenuBar(mb);        Menu m1 = new Menu("文件");        Menu m2 = new Menu("帮助");        mb.add(m1);        mb.add(m2);        MenuItem mi11 = new MenuItem("保存结果");        MenuItem mi12 = new MenuItem("退出");        MenuItem mi2 = new MenuItem("关于计算器");        m1.add(mi11);        m1.add(mi12);        m2.add(mi2);        mi11.addActionListener(new Mb1());        mi12.addActionListener(new Mb1());        mi2.addActionListener(new Mb2());        this.addWindowListener(new WindowAdapter() {            public void windowClosing(WindowEvent e) {                System.exit(0);            }        });    }    public void Draw() {        String str[] = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "+",                "-", "*", "/", ".", "%", "C", "=", };        for (int i = 0; i < b.length; i++) {            b[i] = new Button(str[i]);            jp.add(b[i]);            b[i].addActionListener(new Btn());        }        this.add(tf, BorderLayout.NORTH);    }    public class Btn implements ActionListener {        public void actionPerformed(ActionEvent e) {            String s = e.getActionCommand();            if ("0123456789".indexOf(s) != -1) {                if (flag == 1) {                    flag = 2;//第二次输入数字的时候修改flag的值,影响=号                    valueString = "0";                }                if (s.equals(".") && valueString.indexOf(".") == -1) {                    valueString += s;                } else {                    valueString += s;//数字赋值                    }                if (valueString.startsWith("0")) {                    valueString = valueString.substring(1);//覆盖初始时的0                }            } else if ("+-*/".indexOf(s) != -1) {                if (flag == 0) {                    flag = 1;//第一次输入数字修改flag的值                    value = Double.parseDouble(valueString);//value里保存valueString里第一次输入的数值            } else if (flag == 2) {                    jisuan(valueString, value, fuhao);                    flag = 1;                    }                fuhao = s;            } else if ("=".equals(s)) {                if (flag == 2) {                    jisuan(valueString, value, fuhao);                }                flag = 0;//初始化            } else if ("C".equals(s)) {                value = 0.0;                fuhao = "";                valueString = "0";                flag = 0;            }            showValueString();                    }    }    private void showValueString() {        tf.setText(valueString);            }    private void jisuan(String valueString, double value, String fuhao) {        double v = Double.parseDouble(valueString);        double sum = 0;        if (fuhao == "+") {            sum = v + value;        } else if (fuhao == "-") {            sum = v - value;        } else if (fuhao == "*") {            sum = v * value;        } else if (fuhao == "/") {            sum = v % value;        }        valueString = sum + "";        System.out.println(sum);    }    class Mb1 implements ActionListener {        public void actionPerformed(ActionEvent e) {            System.exit(0);        }    }    class Mb2 extends JFrame implements ActionListener {        public void actionPerformed(ActionEvent e) {            JOptionPane.showMessageDialog(null,                    "", "信息",                    JOptionPane.PLAIN_MESSAGE);        }    }}