3, Simple Expression

import java.math.BigDecimal;
import java.text.DecimalFormat;
import java.util.Scanner;



public class _03_Problem3 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String expression = sc.nextLine().replaceAll(" ", "");

        double totalSum = 0;

        for(int i = 0; i < expression.length(); ) {

            int signPos = nextSignPossition(expression, 1);
            int nextSignPost = nextSignPossition(expression, signPos + 1);
            if(nextSignPost == -1) {
                double firstNumber = Double.parseDouble(expression.substring(0, signPos));
                String operation = expression.substring(signPos, signPos + 1);
                double secondNumber = Double.parseDouble(expression.substring(signPos + 1, expression.length()));

                totalSum = calc(firstNumber, secondNumber, operation);

                break;
            }
            double firstNumber = Double.parseDouble(expression.substring(0, signPos));
            String operation = expression.substring(signPos, signPos + 1);
            double secondNumber = Double.parseDouble(expression.substring(signPos + 1, nextSignPost));

            String sum = calc(firstNumber, secondNumber, operation) + "";

            expression = sum  + expression.substring(nextSignPost);

        }



    }

    public static double calc(double firstValue, double secondValue, String sign) {
        double sum = 0;
        switch (sign) {
        case "+":
            sum = firstValue + secondValue;
            break;
        case "-":
            sum = firstValue - secondValue;
            break;
        default:
            break;
        }

        return sum;
    }

    public static int nextSignPossition (String expression, int start) {
        int signPos;
        int plusIndex = expression.indexOf("+", start);
        int minusIndex = expression.indexOf("-", start);

        if (plusIndex == -1 && minusIndex == -1) {
            signPos = -1;
        }
        else {
            signPos = (plusIndex < minusIndex && plusIndex != -1 || minusIndex == -1) ? plusIndex : minusIndex;
        }

        return signPos;

    }


}