Use character wrapper to code a replace a letter in a string with a new letter keeping the original case

Question

Trying to figure out how to use character wrapping to mutate a string based on user input. If string is \'Bob loves to build building\' and user chooses to replace all letter B/

Answer 1

A simple start is to know about String.replace(char, char) in Java.

// This addresses the example you gave in your question.
str.replace('B', 'T').replace('b', 't');

Then, you should take the user input into toReplace and replaceWith characters, figure out the uppercase/lowercase counter parts using the ASCII code and generate the arguments for the above replace method call.

public class Main
{
    public static void main(String[] arg) throws JSONException
    {
        String str = "Bob loves to build building";
        Scanner scanner = new Scanner(System.in);
        char toReplace = scanner.nextLine().trim().charAt(0);
        char replaceWith = scanner.nextLine().trim().charAt(0);

        System.out.println(str.replace(getUpper(toReplace), getUpper(replaceWith)).replace(getLower(toReplace),
            getLower(replaceWith)));
    }

    private static char getUpper(char ch)
    {
        return (char) ((ch >= 'A' && ch <= 'Z') ? ch : ch - ('a' - 'A'));
    }

    private static char getLower(char ch)
    {
        return (char) ((ch >= 'A' && ch <= 'Z') ? ch + ('a' - 'A') : ch);
    }
}