How can I transform this into better java code

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Java

how can i transform this into better code so i can have more then 1 +'s - or * currently i can only use 1 + so i cant do this 4+4+5 it will crash can anyone help me out,also, Things like 2+1 x 3 works but not things lke 3 x 2 + 3 thats really weird aswell this is the piece of code that it is about,

private static int operatorCount(String formule) {
       int resultaat = 0;

       for (int c =0; c<operators.length; c++) {
           if (formule.indexOf(operators[c]) >= 0) {
               resultaat++;
           }
       }
       return resultaat;
   }

What I have tried:

this is my whole code i aswell have a other file but that are just the formulas,

public class FormuleBereken {
    static char[] operators = {'*', '+','-'};

    static Integer bereken(String formule) {

        formule = cleanUp(formule);
        int resultaat = 0;

        for(int i = 0; i< operators.length; i++) {
            int position = formule.indexOf(operators[i]);

            if (position > -1) {
                int start = position-1;
                while ((start>0)&&(Character.isDigit(formule.charAt(start-1)))) {
                    start --;
                }

                String str1 = formule.substring(start, position);
                String str2 = formule.substring(position + 1);
                String subFormule = formule.substring(start);
                System.out.println(str1);

                //ERROR: Want hij geeft een string mee "2+1" en dat kan niet
                int value1 = Integer.parseInt(str1);
                int value2 = Integer.parseInt(str2);
                switch( operators[i]) {
                    case '*': resultaat = value1 * value2; break;
                    case '+': resultaat = value1 + value2; break;
                    case '-': resultaat = value1 - value2; break;
                }

                formule = formule.replace(subFormule, Integer.toString(resultaat));
                if (operatorCount(formule)!=0) {
                    resultaat = bereken(formule);
                }

            }
        }

        return resultaat;
    }

    // EM: deze geeft nog niet  het goede resultaat , want hij telt elke operator  maar een keer. BDV :dubbele for loop?
    private static int operatorCount(String formule) {
        int resultaat = 0;

        for (int c =0; c<operators.length; c++) {
            if (formule.indexOf(operators[c]) >= 0) {
                resultaat++;
            }
        }
        return resultaat;
    }



    private static String cleanUp(String formule) {
        String newFormule = formule.replaceAll(" ", "");

        return newFormule;
    }
}

if anyone could help me out it will be very appreciated Thanks.