StringCalculator.java

package calculator;

public class StringCalculator {

    public static int calculate(String input) {
        validateNotBlank(input);

        String[] values = input.split(" ");
        return calculate(values);
    }

    private static void validateNotBlank(String input) {
        if (input == null || input.isEmpty()) {
            throw new IllegalArgumentException("Input cannot be null or empty");
        }
    }

    private static int calculate(String[] values) {
        int result = Integer.parseInt(values[0]);

        for (int i = 1; i < values.length; i++) {
            String operation = values[i];
            i++;
            int number = Integer.parseInt(values[i]);
            result = calculate(result, operation, number);
        }

        return result;
    }

    private static int calculate(int number1, String operation, int number2) {
        switch (operation) {
            case "+":
                return number1 + number2;
            case "-":
                return number1 - number2;
            case "*":
                return number1 * number2;
            case "/":
                return number1 / number2;
            default:
                throw new IllegalArgumentException("Invalid operation: " + operation);
        }
    }
}