This is my implementation for the classic code breaking game "Bulls and Cows", a Jet Brains Academy project (stage 7/7)

How program works:

My code:

import java.util.Arrays;

import java.util.Random;

import java.util.Scanner;

public class Main {

static int turn = 1;
static int cows = 0;
static int bulls = 0;
static Scanner scan = new Scanner(System.in);
static String secretCode;
static int digits;
static int symbols;
static Random random = new Random();
static int charRange = 0;
static int maxNum = 9;
static boolean isValidInput = true;

public static void main(String[] args) {
    System.out.println("Input the length of the secret code:");
    digits = checkInput(scan.nextLine());
    if (isValidInput) {
        System.out.println("Input the number of possible symbols in the code:");
        symbols = checkInput(scan.nextLine());
    }
    if (isValidInput) {
        if (parseValues()) {
            secretCode = generateCode(digits, symbols);
            playGame();
        }
    }
}

public static int checkInput(String scanned) {
    boolean numeric;
    numeric = scanned.matches("\\d+");
    if (!numeric) {
        System.out.printf("Error: \"%s\" isn't a valid number.", scanned);
        isValidInput = false;
        return 0;
    } else {
        int aux = Integer.parseInt(scanned);
        if(aux > 36) {
            System.out.println("Error: maximum number of possible symbols" +
                    " in the code is 36 (0-9, a-z).");
            isValidInput = false;
            return 0;
        } else if(aux == 0) {
            System.out.println("Error: minimum number of possible symbols" +
                    " in the code is 1 (0-9, a-z).");
            isValidInput = false;
            return 0;
        }
    }
    return Integer.parseInt(scanned);
}

    public static boolean parseValues() {
        if (digits > symbols) {
            System.out.printf("Error: it's not possible to generate a code with " +
                    "a length of %d with %d unique symbols.", digits, symbols);
            return false;
        }
            return true;
    }

    public static String processGuess () {
        StringBuilder sb = new StringBuilder("Grade: ");
        System.out.printf("Turn %d:\n", turn);
        String guess = scan.next();
        System.out.println("guess: " + guess);
        String[] guessArray = guess.split("");
        System.out.println("guessArray: " + Arrays.toString(guessArray));

        String[] secretCodeArray = secretCode.split("");
        for (int i = 0; i < guessArray.length; i++) {
            if (guessArray[i].equals(secretCodeArray[i])) {
                bulls++;
            }
            if (secretCode.contains(guessArray[i])) {
                ++cows;
            }
        }
        cows = cows - bulls;
        if (bulls > 0) {
            sb.append(String.format("%d bull(s)", bulls));
        }
        if (bulls > 0 && cows > 0) {
            sb.append(" and ");
        }
        if (cows > 0) {
            sb.append(String.format("%d cow(s)", cows));
        }

        if (bulls == 0 && cows == 0) {
            sb.append("none");
        }
        if (bulls == digits) {

            sb.append("\nCongratulations! You guessed the secret code.");
            scan.close();
            return sb.toString();
        }
        bulls = 0;
        cows = 0;
        return sb.toString();
    }

    public static void playGame () {
        String stars = "*".repeat(digits);
        System.out.printf("The secret is prepared: %s (0-%d, a-%c).\n", stars, maxNum, (charRange + 96));
        System.out.println("Okay, let's start a game!");
        System.out.println("digits: " + digits);
        System.out.println("secretCode: " + secretCode);

        while (bulls != digits) {
            System.out.println(processGuess());
            turn++;
        }
    }

    public static String generateCode ( int digits, int symbols){
        System.out.println("digits: " + digits);
        System.out.println("symbols: " + symbols);
        StringBuilder sb = new StringBuilder();
        charRange = symbols - 10;
        if (symbols < 10) {
            maxNum = symbols - 1;
        }
        System.out.println("charRange: " + charRange);
        int charInputs = 0;
        if (charRange > 0) {
            charInputs = random.nextInt(digits + 1);
        }

        System.out.println("charInputs: " + charInputs);
        int numberInputs = digits - charInputs;
        System.out.println("numberInputs: " + numberInputs);

        if (numberInputs == 1) {
            sb.append(random.nextInt(10));
        }
        if (numberInputs > 1) {
            String aux = pseudoRandomNumber().substring(0, numberInputs);
            int[] sortedNumber = sortNumber(aux, numberInputs);
            boolean hasUniqueDigits = false;
            int count = 0;
            while (!hasUniqueDigits) {
                for (int i = 1; i < numberInputs; i++) {
                    if (sortedNumber[i - 1] == sortedNumber[i]) {
                        aux = pseudoRandomNumber().substring(0, numberInputs);
                        sortedNumber = sortNumber(aux, numberInputs);
                        i = 0;
                    } else {
                        count++;
                        if (count == numberInputs - 1) {
                            hasUniqueDigits = true;
                        }
                    }
                }
            }
            sb.append(aux);
        }
        System.out.println("sb.length(): " + sb.length());


        if (charRange > 0) {
            for (int i = 0; i < charInputs; i++) {
                char charAux2 = (char) (random.nextInt(charRange) + 97);
                while (sb.toString().contains(String.valueOf(charAux2))) {
                    charAux2 = (char) (random.nextInt(charRange) + 97);
                }
                sb.insert(random.nextInt(sb.length() != 0 ? sb.length() : 1), charAux2);
            }
        }

        return sb.toString();
    }
    public static int[] sortNumber (String toSort,int digits){
        int[] sortedNumber = new int[digits];
        for (int i = 0; i < digits; i++) {
            sortedNumber[i] = Integer.parseInt(String.valueOf(toSort.charAt(i)));
        }
        Arrays.sort(sortedNumber);
        return sortedNumber;
    }


    public static String pseudoRandomNumber () {
        return String.valueOf(Math.abs(random.nextLong()));
    }

}