How to parse a currency Amount (US or EU) to float value in Java

Question

In Europe decimals are separated with ',' and we use optional '.' to separate thousands. I allow currency values with:

• US-style 123,456.78 notation
• European-style 123.456,78 notation

I use the next regular expression (from RegexBuddy library) to validate the input. I allow optional two-digits fractions and optional thousands separators.

^[+-]?[0-9]{1,3}(?:[0-9]*(?:[.,][0-9]{0,2})?|(?:,[0-9]{3})*(?:\.[0-9]{0,2})?|(?:\.[0-9]{3})*(?:,[0-9]{0,2})?)$

I would like to parse a currency string to a float. For example

123,456.78 should be stored as 123456.78
123.456,78 should be stored as 123456.78
123.45 should be stored as 123.45
1.234 should be stored as 1234 12.34 should be stored as 12.34

and so on...

Is there an easy way to do this in Java?

public float currencyToFloat(String currency) {
    // transform and return as float
}

Use BigDecimal instead of Float

---

Thanks to everyone for the great answers. I have changed my code to use BigDecimal instead of float. I will keep previous part of this question with float to prevent people from doing the same mistakes I was gonna do.

Solution

---

The next code shows a function which transforms from US and EU currency to a string accepted by BigDecimal(String) constructor. That it is to say a string with no thousand separator and a point for fractions.

   import java.util.regex.Matcher;
import java.util.regex.Pattern;


public class TestUSAndEUCurrency {

    public static void main(String[] args) throws Exception {       
        test("123,456.78","123456.78");
        test("123.456,78","123456.78");
        test("123.45","123.45");
        test("1.234","1234");
        test("12","12");
        test("12.1","12.1");
        test("1.13","1.13");
        test("1.1","1.1");
        test("1,2","1.2");
        test("1","1");              
    }

    public static void test(String value, String expected_output) throws Exception {
        String output = currencyToBigDecimalFormat(value);
        if(!output.equals(expected_output)) {
            System.out.println("ERROR expected: " + expected_output + " output " + output);
        }
    }

    public static String currencyToBigDecimalFormat(String currency) throws Exception {

        if(!doesMatch(currency,"^[+-]?[0-9]{1,3}(?:[0-9]*(?:[.,][0-9]{0,2})?|(?:,[0-9]{3})*(?:\\.[0-9]{0,2})?|(?:\\.[0-9]{3})*(?:,[0-9]{0,2})?)$"))
                throw new Exception("Currency in wrong format " + currency);

        // Replace all dots with commas
        currency = currency.replaceAll("\\.", ",");

        // If fractions exist, the separator must be a .
        if(currency.length()>=3) {
            char[] chars = currency.toCharArray();
            if(chars[chars.length-2] == ',') {
                chars[chars.length-2] = '.';
            } else if(chars[chars.length-3] == ',') {
                chars[chars.length-3] = '.';
            }
            currency = new String(chars);
        }

        // Remove all commas        
        return currency.replaceAll(",", "");                
    }

    public static boolean doesMatch(String s, String pattern) {
        try {
            Pattern patt = Pattern.compile(pattern, Pattern.CASE_INSENSITIVE);
            Matcher matcher = patt.matcher(s);
            return matcher.matches();
        } catch (RuntimeException e) {
            return false;
        }           
    }  

}

Answer 1

To answer a slightly different question: don't use the float type to represent currency values. It will bite you. Use a base-10 type instead, like BigDecimal, or an integer type like int or long (representing the quantum of your value - penny, for example, in US currency).

You will not be able to store an exact value - 123.45, say, as a float, and mathematical operations on that value (such as multiplication by a tax percentage) will produce rounding errors.

Example from that page:

float a = 8250325.12f;
float b = 4321456.31f;
float c = a + b;
System.out.println(NumberFormat.getCurrencyInstance().format(c));
// prints $12,571,782.00 (wrong)

BigDecimal a1 = new BigDecimal("8250325.12");
BigDecimal b1 = new BigDecimal("4321456.31");
BigDecimal c1 = a1.add(b1);
System.out.println(NumberFormat.getCurrencyInstance().format(c1));
// prints $12,571,781.43 (right)

You don't want to muck with errors when it comes to money.

With respect to the original question, I haven't touched Java in a little while, but I know that I'd like to stay away from regex to do this kind of work. I see this recommended; it may help you. Not tested; caveat developer.

try {
    String string = NumberFormat.getCurrencyInstance(Locale.GERMANY)
                                            .format(123.45);
    Number number = NumberFormat.getCurrencyInstance(locale)
                                            .parse("$123.45");
    // 123.45
    if (number instanceof Long) {
       // Long value
    } else {
       // too large for long - may want to handle as error
    }
} catch (ParseException e) {
// handle
}

Look for a locale with rules that match what you expect to see. If you can't find one, use multiple sequentially, or create your own custom NumberFormat.

I'd also consider forcing users to enter values in a single, canonical format. 123.45 and 123.456 look way too similar for my tastes, and by your rules would result in values that differ by a factor of 1000. This is how millions are lost.

Answer 2

As a generalized solution you can try

char[] chars = currency.toCharArray();
chars[currency.lastIndexOf(',')] = '.';
currency = new String(chars);

instead of

if(currency.length()>=3) {
    char[] chars = currency.toCharArray();
    if(chars[chars.length-2] == ',') {
        chars[chars.length-2] = '.';
    } else if(chars[chars.length-3] == ',') {
        chars[chars.length-3] = '.';
    }
    currency = new String(chars);
}

so that fractional part can be of any length.