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1. Overview

In this tutorial, we'll see how we can convert a monetary amount into words-representation in Java.

We'll also see how a custom implementation could look like, via an external library – Tradukisto.

2. Implementation

Let's first start with our own implementation. The first step is to declare two String arrays with the following elements:

public static String[] ones = { 
  "", "one", "two", "three", "four", 
  "five", "six", "seven", "eight", 
  "nine", "ten", "eleven", "twelve", 
  "thirteen", "fourteen", "fifteen", 
  "sixteen", "seventeen", "eighteen", 
  "nineteen" 
};

public static String[] tens = {
  "",          // 0
  "",          // 1
  "twenty",    // 2
  "thirty",    // 3
  "forty",     // 4
  "fifty",     // 5
  "sixty",     // 6
  "seventy",   // 7
  "eighty",    // 8
  "ninety"     // 9
};

When we receive an input, we'll need to handle the invalid values (zero and negative values). Once a valid input is received, we can extract the number of dollars and cents into variables:

 long dollars = (long) money;
 long cents = Math.round((money - dollars) * 100);

If the number given is less than 20, then we'll get the appropriate ones' element from the array based on the index:

if (n < 20) {
    return ones[(int) n];
}

We'll use a similar approach for numbers less than 100, but now we have to use tens array as well:

if (n < 100) {
    return tens[(int) n / 10] 
      + ((n % 10 != 0) ? " " : "") 
      + ones[(int) n % 10];
}

We do this similarly for numbers that are less than one thousand.

Next, we use recursive calls to deal with numbers that are less than one million, as shown below:

if (n < 1_000_000) {
    return convert(n / 1000) + " thousand" + ((n % 1000 != 0) ? " " : "") 
      + convert(n % 1000);
}

The same approach is used for numbers that are less than one billion, and so on.

Here is the main method that can be called to do this conversion:

 public static String getMoneyIntoWords(double money) {
    long dollars = (long) money;
    long cents = Math.round((money - dollars) * 100);
    if (money == 0D) {
        return "";
    }
    if (money < 0) {
        return INVALID_INPUT_GIVEN;
    }
    String dollarsPart = "";
    if (dollars > 0) {
        dollarsPart = convert(dollars) 
          + " dollar" 
          + (dollars == 1 ? "" : "s");
    }
    String centsPart = "";
    if (cents > 0) {
        if (dollarParts.length() > 0) {
            centsPart = " and ";
        }
        centsPart += convert(cents) + " cent" + (cents == 1 ? "" : "s");
    }
    return dollarsPart + centsPart;
}

Let's test our code to make sure it works:

@Test
public void whenGivenDollarsAndCents_thenReturnWords() {
    String expectedResult
     = "nine hundred twenty four dollars and sixty cents";
    
    assertEquals(
      expectedResult, 
      NumberWordConverter.getMoneyIntoWords(924.6));
}

@Test
public void whenTwoBillionDollarsGiven_thenReturnWords() {
    String expectedResult 
      = "two billion one hundred thirty three million two hundred" 
        + " forty seven thousand eight hundred ten dollars";
 
    assertEquals(
      expectedResult, 
      NumberWordConverter.getMoneyIntoWords(2_133_247_810));
}

@Test
public void whenThirtyMillionDollarsGiven_thenReturnWords() {
    String expectedResult 
      = "thirty three million three hundred forty eight thousand nine hundred seventy eight dollars";
    assertEquals(
      expectedResult, 
      NumberWordConverter.getMoneyIntoWords(33_348_978));
}

Let's also test some edge cases, and make sure we have covered them as well:

@Test
public void whenZeroDollarsGiven_thenReturnEmptyString() {
    assertEquals("", NumberWordConverter.getMoneyIntoWords(0));
}

@Test
public void whenNoDollarsAndNineFiveNineCents_thenCorrectRounding() {
    assertEquals(   
      "ninety six cents", 
      NumberWordConverter.getMoneyIntoWords(0.959));
}
  
@Test
public void whenNoDollarsAndOneCent_thenReturnCentSingular() {
    assertEquals(
      "one cent", 
      NumberWordConverter.getMoneyIntoWords(0.01));
}

3. Using a Library

Now that we've implemented our own algorithm, let's do this conversion by using an existing library.

Tradukisto is a library for Java 8+, which can help us convert numbers to their word representations. First, we need to import it into our project (the latest version of this library can be found here):

<dependency>
    <groupId>pl.allegro.finance</groupId>
    <artifactId>tradukisto</artifactId>
    <version>1.0.1</version>
</dependency>

We can now use MoneyConverters‘s asWords() method to do this conversion:

public String getMoneyIntoWords(String input) {
    MoneyConverters converter = MoneyConverters.ENGLISH_BANKING_MONEY_VALUE;
    return converter.asWords(new BigDecimal(input));
}

Let's test this method with a simple test case:

@Test
public void whenGivenDollarsAndCents_thenReturnWordsVersionTwo() {
    assertEquals(
      "three hundred ten £ 00/100", 
      NumberWordConverter.getMoneyIntoWords("310"));
}

We could also use the ICU4J library to do this, but it's a large one and comes with many other features that are out of the scope of this article.

However, have a look at it if Unicode and globalization support is needed.

4. Conclusion

In this quick article, we saw two approaches on how to do the conversion of a sum of money into words.

The code for all the examples explained here, and much more can be found over on GitHub.

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