加法分数计算器的运算符

Question

我正在写一个分数计算器。我已经编写了一个解析代码的方法，但是很难弄清楚如何将plus运算符添加到parseExpression方法中。例如：1_1 / 2 + 1 = 2_1 / 2。如果你选择回答尽力让我指向正确的方向。如果有人对减号，除法和乘法运算符有任何建议，我也会很感激。

public class FracCalc {

    public static void main(String[] args) {
        // TODO: Implement.
        Scanner console = new Scanner(System.in);
        System.out.println("enter an equation: ");
        String myInput = console.nextLine();
        String simplified = parseExpression(myInput);
        System.out.print(simplified);
        shouldQuit(myInput);
        console.close();
    }
    public static boolean shouldQuit(String input) {
       String quit = input;
       if (input.equals("quit")) {
       System.out.print("Quit");
       return true;
       }   else {
       return false;
    }
    }
public static String parseExpression(String expr) {
    //parses the expression for '_'
    if(expr.indexOf("_")!=-1) { 
    int spot = expr.indexOf("_");
    String first = expr.substring(0, spot);
    int frac = expr.indexOf("/");
    String numer = expr.substring(spot + 1, frac);
    String denom = expr.substring(frac + 1);
    String mixednum = improperMNum(numer, denom);
    String mixed = first + "_" + mixednum;
    return mixed;
    }
    //parses the expression for '/'
    if (expr.contains("/")) {
    int index = expr.indexOf("/");
    String numerator = expr.substring(0, index);
    String denominator = expr.substring(index + 1);
    System.out.println(numerator + denominator);
    String x = improperMNum(numerator, denominator);
    return x;
    } else {
    String wholenumber = expr.substring(0);
    return wholenumber;
        }
     }
public static String improperMNum(String numerator, String denominator) {
    int wholenumber = numerator.indexOf("_");