Java Program to accept a String and convert all lower case letters to upper case and upper case to lower

The following Code is to convert all the lower case letters to upper and upper case to lower. 'StringBuffer' Class is used because 'String' class is immutable but the 'StringBuffer' class is mutable. Which means that once the value stored in the 'String' type variable, it's value can't be changed but in 'StringBuffer' it can be changed.
Code:-

import java.util.*;
public class CaseConversion {
    int i;
    String string;
    char casse;
    String temp;
    Scanner scan = new Scanner(System.in);
    public void input() {
        try {
            System.out.println("Enter any String: ");
            string = scan.nextLine();
        } catch (Exception e) {
            System.out.println("Error Code: " + e);
            System.exit(0);
        }
    }
    public void compute() {
        StringBuffer conversion = new StringBuffer(string);
        for (i = 0; i < string.length(); i++) {
            casse = string.charAt(i);
            temp = Character.toString(casse);
            if (isUpperCase(casse)) {
                casse = temp.toLowerCase().charAt(0);
                conversion.setCharAt(i, casse);
            } else {
                casse = temp.toUpperCase().charAt(0);
                conversion.setCharAt(i, casse);
            }
        }
        System.out.println("\nString you entered: " + string);
        System.out.println("After Conversion: " + conversion);
    }
    public boolean isUpperCase(char tmp) {
        boolean upper = false;
        if (tmp >= 'A' && tmp <= 'Z') {
            upper = true;
        }
        return upper;
    }
    public static void main(String[] args) {
        CaseConversion obj = new CaseConversion();
        obj.input();
        obj.compute();
    }
}
Output:-
Enter any String:
ConverSION
String you entered: ConverSION
After Conversion: cONVERsion