我的计算器不会乘法或除法

my calculator doesnot multiply or divide numbers

我做了一个不会乘除的计算器 给我一个例外:

java.lang.NumberFormatException: For input string: "*7"

正负没有问题

class ActionListeners implements ActionListener {

    @Override
    public void actionPerformed(ActionEvent e) {
        Object o = e.getSource();
        int n1 = 0;
        int n2 = 0;
        int indexno = 0;
        if (o == result) {

            String s = t1.getText();

            String s3 = "";
            String s4 = "";
            for (int b1 = 0; b1 < s.length(); ++b1) {
                if (s.charAt(b1) == '+') {
                    indexno = s.indexOf('+');
                    s3 = s.substring(0, indexno);
                    s4 = s.substring(indexno);
                    n1 = Integer.parseInt(s3);
                    n2 = Integer.parseInt(s4);
                    JOptionPane.showMessageDialog(null, n1 + n2);
                }
                if (s.charAt(b1) == '-') {
                    indexno = s.indexOf('-');
                    s3 = s.substring(0, indexno);
                    s4 = s.substring(indexno);
                    n1 = Integer.parseInt(s3);
                    n2 = Integer.parseInt(s4);
                    JOptionPane.showMessageDialog(null, n1 + n2);
                }
                if (s.charAt(b1) == '*') {
                    indexno = s.indexOf('*');
                    s3 = s.substring(0, s.indexOf('*'));
                    s4 = s.substring(indexno);
                    n1 = Integer.parseInt(s3);
                    n2 = Integer.parseInt(s4);
                    JOptionPane.showMessageDialog(null, n1 * n2);
                }
                if (s.charAt(b1) == '/') {
                    indexno = s.indexOf('/');
                    s3 = s.substring(0, s.indexOf('/'));
                    s4 = s.substring(indexno);
                    n1 = Integer.parseInt(s3);
                    n2 = Integer.parseInt(s4);
                    JOptionPane.showMessageDialog(null, n1 / n2);
                }
                if (s.charAt(b1) == '/' && n2 == 0) {
                    JOptionPane.showMessageDialog(null, "Error : cant divide by zero");
                }

            }

            t1.setText("");
        }

- 起作用的唯一原因是 JOptionPane.showMessageDialog(null, n1 + n2);...请注意,您正在解析 n2 中的一元负值,然后添加两个值;如果你做了JOptionPane.showMessageDialog(null, n1 - n2);,你也会因为减法中的一个错误而被淘汰。您希望每个 if(s) 中有 s4 = s.substring(indexno + 1);