Username

package final_exam_prep;

import java.util.Scanner;

public class Username {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        String input = scanner.nextLine();
        String command = scanner.nextLine();
        while (!command.equals("Sign up")) {
            String[] commands = command.split("\\s+");
            switch (commands[0]) {

                case "Case": {
                    if (commands[1].equals("upper")) {
                        input = input.toUpperCase();
                        System.out.println(input);
                    } else if (commands[1].equals("lower")){
                        input = input.toLowerCase();
                        System.out.println(input);
                    } else {
                        continue;
                    }
                }
                break;

                case "Reverse": {
                    int start = Integer.parseInt(commands[1]);
                    int end = Integer.parseInt(commands[2]);
                    String substring = "";
                    if (notOutOfBounds(start, end + 1, input.length())) {
                        substring = input.substring(start, end + 1);

                        StringBuilder sb = new StringBuilder();
                        for (int i = substring.length() - 1; i >= 0; i--) {
                            String letter = String.valueOf(substring.charAt(i));
                            sb.append(letter);
                        }
                        System.out.println(sb.toString());
                    }
                }
                break;

                case "Cut": {
                    String substring = commands[1];
                    if (input.contains(substring)) {
                        System.out.println(input.replace(substring, ""));
                    } else {
                        System.out.printf("The word %s doesn't contain %s.%n", input, substring);
                    }
                }
                break;

                case "Replace": {
                    char symbol = commands[1].charAt(0);
                    input = input.replaceAll(String.valueOf(symbol), "*");
                    System.out.println(input);
                }
                break;

                case "Check": {
                    char symbol = commands[1].charAt(0);
                    if (input.contains(String.valueOf(symbol))) {
                        System.out.println("Valid");
                    } else {
                        System.out.printf("Your username must contain %c.%n", symbol);
                    }
                }
                break;

            }
            command = scanner.nextLine();
        }
    }

    public static boolean notOutOfBounds(int index, int index2, int size) {
        if (index >= 0 && index < size && index2 >= 0 && index2 < size) {
            return true;
        }
        return false;
    }
}