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Thread: Switching letter-cases from user input; why is not working?

  1. #1
    Junior Member Kimiko Sakaki's Avatar
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    Default Switching letter-cases from user input; why is not working?

    Hey guys,

    So I just started learning Java this year so you can say I'm fresh off the boat. Though I've learned programming concepts before (from Turing). Right now, I'm experiencing some difficulty in comparing/checking if a certain character is upper-case or lower-case.

    This simple program just asks for the user to input a word/phrase and whatever letter is lower-case, I change it to upper-case and whatever letter is upper-case, I change it to lower-case.

    import java.util.*;
    class UpperLower {
        public static void main() {
            String userInput;
     
            Scanner input = new Scanner (System.in );
            System.out.println ("Please enter something...:");
            userInput = input.nextLine();
            String copyInput = userInput;
     
            for (int num = 0; num < userInput.length(); num++) {
                if (Character.isUpperCase(userInput.charAt(num))){
                    (userInput.substring(num)).toLowerCase();
                } else {
                    (userInput.substring(num)).toUpperCase();
                }
            }
     
            System.out.println (userInput);
        }
    }

    There seemed to be no compiler error however, when I tested my program, the letters' case were still unchanged. I'm not sure what is wrong with my code (perhaps it has to do with my if-condition?).

    Please give this a try and see what is wrong. I really do appreciate your efforts, time and help. Thank you very much in advance!


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    Super Moderator helloworld922's Avatar
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    Default Re: Switching letter-cases from user input; why is not working?

    Strings in Java are immutable. Thus, every time you make a change to them you're actually creating a new string with that change. This can be remedied by re-assigning back the new string to the old variable (be careful that you get the whole string and not just the part you changed).

    If you want a mutable version of a String, use the StringBuilder class.

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    Junior Member Kimiko Sakaki's Avatar
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    Default Re: Switching letter-cases from user input; why is not working?

    Thank you for the help! I'll try and see what I can do with the StringBuilder class.

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