Check a given sentence for a given set of simple grammar rules

// Java program to validate a given sentence 
// for a set of rules
class GFG 
{

    // Method to check a given sentence for given rules
    static boolean checkSentence(char[] str)
    {

        // Calculate the length of the string.
        int len = str.length;

        // Check that the first character lies in [A-Z].
        // Otherwise return false.
        if (str[0] < 'A' || str[0] > 'Z')
            return false;

        // If the last character is not a full stop(.) 
        // no need to check further.
        if (str[len - 1] != '.')
            return false;

        // Maintain 2 states. Previous and current state 
        // based on which vertex state you are. 
        // Initialise both with 0 = start state.
        int prev_state = 0, curr_state = 0;

        // Keep the index to the next character in the string.
        int index = 1;

        // Loop to go over the string.
        while (index <= str.length) 
        {

            // Set states according to the input characters 
            // in the string and the rule defined in the description.
            // If current character is [A-Z]. Set current state as 0.
            if (str[index] >= 'A' && str[index] <= 'Z')
                curr_state = 0;

            // If current character is a space. 
            // Set current state as 1.
            else if (str[index] == ' ')
                curr_state = 1;

            // If current character is [a-z]. 
            // Set current state as 2.
            else if (str[index] >= 'a' && str[index] <= 'z')
                curr_state = 2;

            // If current state is a dot(.).
            // Set current state as 3.
            else if (str[index] == '.')
                curr_state = 3;

            // Validates all current state with previous state 
            // for the rules in the description of the problem.
            if (prev_state == curr_state && curr_state != 2)
                return false;
                
            if (prev_state == 2 && curr_state == 0)
                return false;

            // If we have reached last state and previous state 
            // is not 1, then check next character. If next character 
            // is '\0', then return true, else false
            if (curr_state == 3 && prev_state != 1)
                return (index + 1 == str.length);

            index++;

            // Set previous state as current state 
            // before going over to the next character.
            prev_state = curr_state;
        }
        return false;
    }

    // Driver Code
    public static void main(String[] args)
    {
        String[] str = { "I love cinema.", "The vertex is S.", 
                         "I am single.", "My name is KG.", 
                         "I lovE cinema.", "GeeksQuiz. is a quiz site.", 
                         "I love Geeksquiz and Geeksforgeeks.", 
                         " You are my friend.", "I love cinema" };
        int str_size = str.length;

        int i = 0;
        for (i = 0; i < str_size; i++)
        {
            if (checkSentence(str[i].toCharArray()))
                System.out.println("\"" + str[i] + 
                                   "\"" + " is correct");
            else
                System.out.println("\"" + str[i] + 
                                   "\"" + " is incorrect");
        }
    }
}

// This code is contributed by
// sanjeev2552