Username - String Manipulation

package com.company;

import javafx.util.Pair;

import javax.xml.crypto.dsig.keyinfo.KeyValue;
import java.security.KeyStore;
import java.text.Collator;
import java.text.DecimalFormat;
import java.util.*;
import java.util.function.Predicate;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.Collector;
import java.util.stream.Collectors;


public class Main {

    public static String replaceCase(StringBuilder builder, String command){
        String result = "";
        if (command.equals("lower")){
            result = builder.toString().toLowerCase();
        } else if(command.equals("upper")){
            result = builder.toString().toUpperCase();
        }
        System.out.println(result);
        return result;
    }

    public static void printReversed(String username, int startIndex, int endIndex){
        String substring = username.substring(startIndex, endIndex);
        StringBuilder reverseSubstring = new StringBuilder("");
        for (int i = substring.length() - 1; i >= 0; i--) {
            reverseSubstring.append(substring.charAt(i));
        }
        System.out.println(reverseSubstring.toString());
    }

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

        String username = scanner.nextLine();
        StringBuilder builder = new StringBuilder(username);

        String input = "";
        while(!"Sign up".equals(input = scanner.nextLine())){
            String[] tokens = input.split(" ");
            String command = tokens[0];

            switch (command){
                case "Case":
                    String cmd = tokens[1];
                    builder = new StringBuilder(replaceCase(builder, cmd));
                    break;
                case "Reverse":
                    int startIndex = Integer.parseInt(tokens[1]);
                    int endIndex = Integer.parseInt(tokens[2]) + 1;
                    if((startIndex >= 0 && startIndex < builder.length())
                            && (endIndex <= builder.length() && endIndex >= 0)){
                        printReversed(builder.toString(), startIndex, endIndex);
                    }
                    break;
                case "Cut":
                    String substr = tokens[1];
                    if(builder.toString().contains(substr)){
                        builder = new StringBuilder(builder.toString().replace(substr, ""));
                        System.out.println(builder.toString());
                    }else{
                        System.out.printf("The word %s doesn't contain %s.\n", builder.toString(), substr);
                    }
                    break;
                case "Replace":
                    String character = tokens[1] + "";
                    if(builder.toString().contains(character)) {
                        builder = new StringBuilder(builder.toString().replace(character, "*"));
                    }
                    System.out.println(builder.toString());
                    break;
                case "Check":
                    String validChar = tokens[1] + "";
                    if(builder.toString().contains(validChar)) {
                        System.out.println("Valid");
                    }else {
                        System.out.println("Your username must contain " + validChar + ".");
                    }
                    break;
            }
        }
    }
}