Int to Char for arraylist quicksort

Question

Hey, i need help as our teacher was decent but he went through the sorting extremely quickly so it was hard to understand. Therefore i need help changing this to a char which reads Strings. It currently reads GPA(int) but i need it to read last names and first names (string ------> chars) here is the code i have:
import java.util.ArrayList;
public class SortGPA {
     private void SortGPA(ArrayList list, int low0, int high0) {
          int low = low0, high = high0;
          if (low >= high) {
               return;
          } else if (low == high - 1) {
               if (((Comparable)list.get(low)).compareTo(list.get(high)) > 0) {
                    Object temp = list.get(low);
                    list.set(low, list.get(high));
                    list.set(high, temp);
               return;
          Object pivot = list.get((low + high) / 2);
          list.set((low + high) / 2, list.get(high));
          list.set(high, pivot);
          while (low < high) {
               while (((Comparable)list.get(low)).compareTo(pivot) <= 0 && low < high) {
                    low++;
               while (((Comparable)list.get(high)).compareTo(pivot) >= 0 && low < high) {
                    high--;
               if (low < high) {
                    Object temp = list.get(low);
                    list.set(low, list.get(high));
                    list.set(high, temp);
          list.set(high0, list.get(high));
          list.set(high, pivot);
          SortGPA(list, low0, low - 1);
          SortGPA(list, high + 1, high0);
     public void sort(ArrayList list) {
          SortGPA(list, 0, list.size() - 1);
}

Answer 1

And for that matter, use increment/decrement assignment operators (like += and -=) instead of verbosely repeating the variable. And if you're going to use character-by-character case conversion, use the methods in Character don't do it like a C programmer circa 1982. And if this is anything other than a homework assignment, use encryption code provided in the JDK; don't reinvent it.