java - 计算分数表达式 : Subtraction

Question

我正在用java创建一个分数类。该程序采用整个分数表达式，并根据带有 switch 语句的运算符执行操作。我一切正常，除了当程序给出一个减法表达式(即 -1/2--1/2)时。当我测试这个表达式时，我收到一个数字格式异常。我相信这与我的 StringTokenizer 中的分隔符有关，但不知道该怎么做。如有任何帮助，我们将不胜感激。

String regex = "-?[0-9]+[ \t]*/[ \t]*-?[0-9]+[ \t]*[+-/\\*][ \t]*-?[0-9]+[ \t]*/[ \t]*-?[0-9]+";
Scanner input = new Scanner(System.in);
 System.out.print("Please enter a fraction expression i.e., -1/2 - -1/4: ");
        String expression = input.nextLine();

        if (!expression.matches(regex)) {
            System.out.println("Not a valid fraction expression");
        }
ArrayList<String> myList = new ArrayList<>();
StringTokenizer st = new StringTokenizer(expression, "-/*\t ", true);
        while (st.hasMoreTokens()) {
            String token = st.nextToken();
            if (!token.equals(" ") && !token.equals("\t"))
                myList.add(token);
        }

 int num1 = Integer.parseInt(myList.get(0));
        int den1 = Integer.parseInt(myList.get(2));
        int num2 = Integer.parseInt(myList.get(4));
        int den2 = Integer.parseInt(myList.get(6));

        Fraction fraction1 = new Fraction(num1, den1);
        Fraction fraction2 = new Fraction(num2, den2);

        String operator = myList.get(3);

switch (operator) {
            case "+":
                System.out.println(fraction1.add(fraction2));
                break;
            case "-":
                System.out.println(fraction1.sub(fraction2));
                break;
            case "*":
                System.out.println(fraction1.mult(fraction2));
                break;
            case "/":
                System.out.println(fraction1.div(fraction2));
                break;
        }

Answer 1

我测试了您的代码片段，并在更改分隔符时使其正常工作 “-/\t”到 “/\t”

另一个注意事项:当输入的字符串与您的表达式不匹配时，您可能希望从代码中返回:

    if (!expression.matches(regex)) {
        System.out.println("Not a valid fraction expression");
        return;
    }