public class NumberToWordsConverter {
 
    private static final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private static final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private static final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};
 
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        int i = 0;
        String words = "";
        while (num > 0) {
            if (num % 1000 != 0) {
                words = helper(num % 1000) + THOUSANDS[i] + " " + words;
            }
            num /= 1000;
            i++;
        }
        return words.trim();
    }
 
    private String helper(int num) {
        if (num == 0) return "";
        else if (num < 20) return LESS_THAN_20[num] + " ";
        else if (num < 100) return TENS[num / 10] + " " + helper(num % 10);
        else return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
    }
 
    public String convert(double amount) {
        long integerPart = (long) amount;
        double decimalPart = amount - integerPart;
 
        String integerWords = numberToWords((int) integerPart);
        String decimalWords = numberToWords((int) (decimalPart * 100));
 
        return integerWords + " Dollars and " + decimalWords + " Cents";
    }
}

import org.junit.Test;
import static org.junit.Assert.assertEquals;
 
public class NumberToWordsConverterTest {
    @Test
    public void testConvert() {
        NumberToWordsConverter converter = new NumberToWordsConverter();
        assertEquals("One Hundred Twenty Three Thousand Four Hundred Fifty Six Dollars and Seventy Eight Cents",
                     converter.convert(123456.78));
        assertEquals("One Dollar and Twenty Three Cents",
                     converter.convert(1.23));
        assertEquals("Zero Dollars and Zero Cents",
                     converter.convert(0.00));
    }
}

import java.util.HashMap;
import java.util.Map;
 
public class NumberToWordsConverter {
 
    private static final String[] unitsMap = {
        "", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",
        "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"
    };
 
    private static final String[] tensMap = {
        "", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"
    };
 
    private static final Map<Long, String> thousandsMap = new HashMap<>();
    static {
        thousandsMap.put(1000000000L, "Billion");
        thousandsMap.put(1000000L, "Million");
        thousandsMap.put(1000L, "Thousand");
        thousandsMap.put(1L, "");
    }
 
    public static void main(String[] args) {
        double amount = 123456.78;
        System.out.println("Amount in words: " + convertNumberToWords(amount));
    }
 
    public static String convertNumberToWords(double number) {
        long integerPart = (long) number;
        int decimalPart = (int) Math.round((number - integerPart) * 100);
        StringBuilder result = new StringBuilder();
 
        if (integerPart == 0) {
            result.append("Zero Dollars");
        } else {
            result.append(convertLessThanOneThousand(integerPart)).append(" Dollars");
        }
 
        if (decimalPart > 0) {
            result.append(" and ").append(convertLessThanOneThousand(decimalPart)).append(" Cents");
        }
 
        return result.toString().trim();
    }
 
    private static String convertLessThanOneThousand(long number) {
        if (number % 100 < 20) {
            return unitsMap[(int) number % 100];
        } else {
            return tensMap[(int) (number % 100 / 10)] + " " + unitsMap[(int) (number % 10)];
        }
    }
 
    private static String convert(long number) {
        if (number == 0) {
            return "";
        }
 
        for (Map.Entry<Long, String> entry : thousandsMap.entrySet()) {
            if (number >= entry.getKey()) {
                return convert(number / entry.getKey()) + " " + entry.getValue() + " " + convert(number % entry.getKey());
            }
        }
        return "";
    }
}