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    public static double evaluate(String expression) {
        Stack<Double> stack = new Stack<>();
        Stack<Character> operatorStack = new Stack<>();

        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);
            if (Character.isDigit(c) || c == '.') {
                StringBuilder sb = new StringBuilder();
                sb.append(c);
                while (i + 1 < expression.length() && (Character.isDigit(expression.charAt(i + 1)) || expression.charAt(i + 1) == '.')) {
                    sb.append(expression.charAt(i + 1));
                    i++;
                }
                stack.push(Double.parseDouble(sb.toString()));
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                while (!operatorStack.isEmpty() && precedence(c) <= precedence(operatorStack.peek())) {
                    double secondOperand = stack.pop();
                    double firstOperand = stack.pop();
                    char operator = operatorStack.pop();
                    stack.push(applyOperator(firstOperand, secondOperand, operator));
                }
                operatorStack.push(c);
            }
        }

        while (!operatorStack.isEmpty()) {
            double secondOperand = stack.pop();
            double firstOperand = stack.pop();
            char operator = operatorStack.pop();
            stack.push(applyOperator(firstOperand, secondOperand, operator));
        }

        return stack.pop();
    }

    private static int precedence(char operator) {
        if (operator == '+' || operator == '-') {
            return 1;
        } else if (operator == '*' || operator == '/') {
            return 2;
        } else {
            return -1;
        }
    }

    private static double applyOperator(double firstOperand, double secondOperand, char operator) {
        if (operator == '+') {
            return firstOperand + secondOperand;
        } else if (operator == '-') {
            return firstOperand - secondOperand;
        } else if (operator == '*') {
            return firstOperand * secondOperand;
        } else {
            return firstOperand / secondOperand;
        }
    }