Something

public String getAlpha(int num) {
int base = 64;
String results = "";


while (num > 0) {
 int quo = Math.floor(num / base);
 int rem = num % base;
 // 'rem' is the # of the character you want. i don't know the best way to do this
 char chr = ...;
 results = results + chr;
 num = quo;
}

return results;
}