Write a program to input a character (ch)convert the character into its opposite case. print the entered character in the new character the program should reject if the elector entered is not an alphabet A to z or a to z your program should continue as long as the user wants.
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import java.util.Scanner;  

import java.lang.*;

class Main {

 public static void main(String[] args) {

   boolean exit = false;

   Scanner reader = new Scanner(System.in);

   while(!exit) {

     char c = '?';

     while( !Character.isLetter(c)) {

       System.out.print("Enter a letter or * to exit: ");

       c = reader.next().charAt(0);

       if (c == '*') {

         exit = true;

         break;

       }

       if (!Character.isLetter(c)) {

         System.out.printf("Error: %c is not a letter!\n", c);

       }

     }

     if (Character.isLowerCase(c)) {

       System.out.printf("%c in uppercase is %c\n", c, Character.toUpperCase(c));

     } else if (Character.isUpperCase(c)) {

       System.out.printf("%c in lowercase is %c\n", c, Character.toLowerCase(c));

     }

   }

   reader.close();

 }

}

Explanation:

I decided to use the asterisk (*) as exit character.