String[] names = new String[]{"", "one", "two", "three", "four", "five", "six", "secen", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "sixteen", "eighteen", "nineteen"};
 
//We don't need names for everything up to nineteen
String[] names10 = new String[]{"", "", "twenty", "thirty", "fourty", "fifty", "sixty", "eighty", "ninety"};
 
String[] groupnames = new String[]{"", "thousand", "million", "billion", ...};
 
//Every one of your groups is a integer from 0 to 999.
//This example should give you an idea how to crate the complete number. It may contain some errors, I didn't compile it.
 
int groupValue = ...;
 
int firstDigit = groupValue / 100; //DIV operation cut's off the rest digits.
int secondDigit = (groupValue / 10) % 10;
int thirdDigit = groupValue % 10;
 
String name = "";
if (firstDigit != 0) {
    name += names[firstDigit] + "hundred";
}
if (secondDigit != 0 || thirdDigit != 0) {
    if (firstDigit != 0) {
        name += " and ";
    }
    if (secondDigit < 1) {
        name += names[secondDigit * 10 + thirdDigit];
    } else {
        name += names10[secondDigit] + names[thirdDigit];
    }
}