public class ExpressionParser {

     public static String[] operators = new String[] {"", "+", "-"};

     public static void main(String []args){
        String numString = "123";
        int sum = 0;
        int length = numString.length();

        // numString on its own is positive
        recursion(0, numString, length, sum);

        String negNumString = operators[2] + numString;

     }

     public static void recursion(int currentIndex, String sub, int length, int sum)
     {

        // Base case
          if (sub.charAt(0) == '-' && length == 2 || length == 1)
          {
             //if (parse(sub) == sum) { System.out.println(sub); }

             // just append _sub, +sub, and -sub, no recursion call
             // and parse them all
          }
          else
          {
             String newSub = sub.substring(currentIndex+1, length);
             // append _, +, - in front of newSub in new blank, plus, minus strings
             // parse each one to see if right value
             // then do recursion(currentIndex+1, whateversub, whateversub.length(), 0);
             String spaceSub = operators[0] + newSub;
             recursion(currentIndex+1, spaceSub, spaceSub.length(), sum);
             String addSub = operators[1] + newSub;
             recursion(currentIndex+1, addSub, addSub.length(), sum); // length + 1?
             String subSub = operators[2] + newSub;
             recursion(currentIndex+1, subSub, subSub.length(), sum); // length + 1?

          }

     }

}