BasicCalculator.java

public int calculate(String s) {
        if (s == null || s.length() == 0) {
            throw new IllegalArgumentException("Invalid input");
        }

        int i = 0;
        Stack<Integer> operands = new Stack<>();
        Stack<Character> operators = new Stack<>();
        StringBuilder number = new StringBuilder();

        while (i < s.length()) {
            char c = s.charAt(i);
            if (c == ' ') {
                i++;
                continue;
            }

            if (Character.isDigit(c)) {
                number.append(c);
            } else if (c == '(') {
                operators.push(c);
            } else if (c == ')') {
                // push the current number to the operands stack
                if (number.length() != 0) {
                    operands.push(Integer.parseInt(number.toString()));
                    number = new StringBuilder();
                }

                if (!operators.isEmpty() && operators.peek() != '(') {
                    char op = operators.pop();
                    operands.push(this.calculateValue(operands, op));
                }
                if (!operators.isEmpty()) { // pop the left bracket
                    operators.pop();
                }
            } else if (c == '+' || c == '-') {
                if (number.length() != 0) {
                    operands.push(Integer.parseInt(number.toString()));
                    number = new StringBuilder();
                }
                
                if (!operators.isEmpty() && operators.peek() != '(') {
                    operands.push(this.calculateValue(operands, operators.pop()));
                }

                operators.push(c);
            }
            i++;
        }


        if (number.length() != 0) {
            operands.push(Integer.parseInt(number.toString()));
        }

        // need this for the 1+1 case, don't need a while since we are only one step away
        if (!operators.isEmpty()) {
            operands.push(this.calculateValue(operands, operators.pop()));
        }

        return operands.pop();
    }


    private int calculateValue(Stack<Integer> operands, char operator) {
        // Pay attention to the stack order
        int o2 = operands.pop();
        int o1 = operands.pop();

        if (operator == '+') {
            return o1 + o2;
        } else if (operator == '-') {
            return o1 - o2;
        } else {
            throw new IllegalArgumentException("invalid operator!");
        }
    }