java - BigDecimal

Question

I was trying to make my own class for currencies using longs, but Apparently I should use BigDecimal (and then whenever I print it just add the $ sign before it). Could someone please get me started? What would be the best way to use BigDecimals for Dollar currencies, like making it at least but no more than 2 decimal places for the cents, etc. The api for BigDecimal is huge, and I don't know which methods to use. Also, BigDecimal has better precision, but isn't that all lost if it passes through a double? if I do new BigDecimal(24.99), how will it be different than using a double? Or should I use the constructor that uses a String instead?

EDIT: I decided to use BigDecimals, and then use:

private static final java.text.NumberFormat moneyt =
        java.text.NumberFormat.getCurrencyInstance();
{
    money.setRoundingMode(RoundingMode.HALF_EVEN);
}

and then whenever I display the BigDecimals, to use money.format(theBigDecimal). Is this alright? Should I have the BigDecimal rounding it too? Because then it doesn't get rounded before it does another operation.. if so, could you show me how? And how should I create the BigDecimals? new BigDecimal("24.99") ?

Well, after many comments to Vineet Reynolds (thanks for keeping coming back and answering), this is what I have decided. I use BigDecimals and a NumberFormat. Here is where I create the NumberFormat instance.

private static final NumberFormat money;
static {
    money = NumberFormat.getCurrencyInstance(Locale.CANADA);
    money.setRoundingMode(RoundingMode.HALF_EVEN);
}

Here is my BigDecimal:

private final BigDecimal price;

Whenever I want to display the price, or another BigDecimal that I got through calculations of price, I use:

money.format(price)

to get the String. Whenever I want to store the price, or a calculation from price, in a database or in a field or anywhere, I use (for a field):

myCalculatedResult = price.add(new BigDecimal("34.58")).setScale(2, RoundingMode.HALF_EVEN);

.. but I'm thinking now maybe I should not have the NumberFormat round, but when I want to display do this:

System.out.print(money.format(price.setScale(2, RoundingMode.HALF_EVEN);

That way to ensure the model and things displayed in the view are the same. I don't do:

price = price.setScale(2, RoundingMode.HALF_EVEN);

Because then it would always round to 2 decimal places and wouldn't be as precise in calculations.

So its all solved now, I guess. But is there any shortcut to typing calculatedResult.setScale(2, RoundingMode.HALF_EVEN) all the time? All I can think of is static importing HALF_EVEN...

EDIT: I've changed my mind a bit, I think if I store a value, I won't round it unless I have no more operations to do with it, e.g. if it was the final total that will be charged to someone. I will only round things at the end, or whenever necessary, and I will still use NumberFormat for the currency formatting, but since I always want rounding for display, I made a static method for display:

public static String moneyFormat(BigDecimal price) {
    return money.format(price.setScale(2, RoundingMode.HALF_EVEN));
}

So values stored in variables won't be rounded, and I'll use that method to display prices.

Answer 1

1) If you are limited to the double precision, one reason to use BigDecimals is to realize operations with the BigDecimals created from the doubles.

2) The BigDecimal consists of an arbitrary precision integer unscaled value and a non-negative 32-bit integer scale, while the double wraps a value of the primitive type double in an object. An object of type Double contains a single field whose type is double

3) It should make no difference

You should have no difficulties with the $ and precision. One way to do it is using System.out.printf

Answer 2

Here are a few hints:

1. Use BigDecimal for computations if you need the precision that it offers (Money values often need this).
2. Use the NumberFormat class for display. This class will take care of localization issues for amounts in different currencies. However, it will take in only primitives; therefore, if you can accept the small change in accuracy due to transformation to a double, you could use this class.
3. When using the NumberFormat class, use the scale() method on the BigDecimal instance to set the precision and the rounding method.

PS: In case you were wondering, BigDecimal is always better than double, when you have to represent money values in Java.

PPS:

Creating BigDecimal instances

This is fairly simple since BigDecimal provides constructors to take in primitive values, and String objects. You could use those, preferably the one taking the String object. For example,

BigDecimal modelVal = new BigDecimal("24.455");
BigDecimal displayVal = modelVal.setScale(2, RoundingMode.HALF_EVEN);

Displaying BigDecimal instances

You could use the setMinimumFractionDigits and setMaximumFractionDigits method calls to restrict the amount of data being displayed.

NumberFormat usdCostFormat = NumberFormat.getCurrencyInstance(Locale.US);
usdCostFormat.setMinimumFractionDigits( 1 );
usdCostFormat.setMaximumFractionDigits( 2 );
System.out.println( usdCostFormat.format(displayVal.doubleValue()) );

Answer 3

Use BigDecimal.setScale(2, BigDecimal.ROUND_HALF_UP) when you want to round up to the 2 decimal points for cents. Be aware of rounding off error when you do calculations though. You need to be consistent when you will be doing the rounding of money value. Either do the rounding right at the end just once after all calculations are done, or apply rounding to each value before doing any calculations. Which one to use would depend on your business requirement, but generally, I think doing rounding right at the end seems to make a better sense to me.

Use a String when you construct BigDecimal for money value. If you use double, it will have a trailing floating point values at the end. This is due to computer architecture regarding how double/float values are represented in binary format.

Answer 4

There is an extensive example of how to do this on javapractices.com. See in particular the Money class, which is meant to make monetary calculations simpler than using BigDecimal directly.

The design of this Money class is intended to make expressions more natural. For example:

if ( amount.lt(hundred) ) {
 cost = amount.times(price); 
}

The WEB4J tool has a similar class, called Decimal, which is a bit more polished than the Money class.

Answer 5

I would recommend a little research on Money Pattern. Martin Fowler in his book Analysis pattern has covered this in more detail.

public class Money {

    private static final Currency USD = Currency.getInstance("USD");
    private static final RoundingMode DEFAULT_ROUNDING = RoundingMode.HALF_EVEN;

    private BigDecimal amount;
    private Currency currency;   

    public static Money dollars(BigDecimal amount) {
        return valueOf(amount, USD);
    }

    Money(BigDecimal amount, Currency currency) {
        this(amount, currency, DEFAULT_ROUNDING);
    }

    Money(BigDecimal amount, Currency currency, RoundingMode rounding) {
        this.amount = amount;
        this.currency = currency;

        this.amount = amount.setScale(currency.getDefaultFractionDigits(), rounding);
    }

    public BigDecimal getAmount() {
        return amount;
    }

    public Currency getCurrency() {
        return currency;
    }

    @Override
    public String toString() {
        return getCurrency().getSymbol() + " " + getAmount();
    }

    public String toString(Locale locale) {
        return getCurrency().getSymbol(locale) + " " + getAmount();
    }   
}

Coming to the usage: You would represent all monies using Money object as opposed to BigDecimal. Representing money as big decimal will mean that you will have the to format the money every where you display it. Just imagine if the display standard changes. You will have to make the edits all over the place. Instead using the Money pattern you centralize the formatting of money to a single location.

Money price = Money.dollars(38.28);
System.out.println(price);