PostfixNotation

// first version
float eval(String str) {
  String [] tokens = str.split(" ");
  Stack <Float> stack = new Stack<Float>();

  for (String token: tokens) {
    if (!isOp(token)) {
  } else {
    float right = stack.remove();
    float left = stack.remove();
    float t = calculate(token, left, right);
    stack.add(t);
  }

  return stack.peek();
}

float calculate(String op, float left, float right) {
  if (op.equals("+")) {
    return left + right;
  } else if (op.equals("-")) {
    return left - right;
  } else if (op.equals("*")) {
    return left * right;
  } else /*if (op.equals("/"))*/  {
    return left / right;
  }
}

boolean isOp(String s) {
   return (s.eqals("+") || s.equals("-") || s.equals("*") || s.equals("/"));
}

// second version
float eval(String str) {
  String [] tokens = str.split(" ");
  Stack <Float> stack = new Stack<Float>();

  boolean invalid = false;
  for (String token: tokens) {
    if (!isOp(token)) {
      try {
        stack.add(Float.parseInt(token));
      } catch (FormattingException e) {
        throw new ArithmeticException(token + " no a valid number");
      }
    } else {
      if (!stack.empty()) {
        float right = stack.remove();
      } else {
        invalid = true;
        break;
      }
           
      if (!stack.empty()) {
        float left = stack.remove();
      } else {
        invalid = true;
        break;
      }
      float t = calculate(token, left, right);
      stack.add(t);
    }
  }
    
  if (invalid || stack.size() > 1 || stack.size() == 0) {
    throw new ArithmeticException("");
  } else {
    return stack.peek();
  }
}

float calculate(String op, float left, float right) {
  if (op.equals("+")) {
    return left + right;
  } else if (op.equals("-")) {
    return left - right;
  } else if (op.equals("*")) {
    return left * right;
  } else /*if (op.equals("/"))*/  {
    if (Math.abs(right) < 1e-9) { // right == 0.0
      throw new ArithmeticException("div by 0");
    }
    return left / right;
  }
}

boolean isOp(String s) {
  return (s.eqals("+") || s.equals("-") || s.equals("*") || s.equals("/"));
}