Dealing With Money

The Java Double (or Float) classes are not suitable for representing money in an accurate way. One of the simplest ways to describe this problem is with a short code snippet:

public class DoubleTest {
    public static void main(String args[]) {
        Double num = 1.2 - 1.1;
        System.out.println(num);
    }
}

Run this code and we get...

0.09999999999999987

Well that's certainly not what we would expect from a mathematical point of view. However, due to the way floating point numbers work, we don't always have absolute precision, especially when addition and subtraction come into the picture.

Always use the Broadleaf class Money, which is a wrapper on top of BigDecimal to represent finances. Also, be sure to instantiate the class like this:

new Money("1.1");

and not like this:

new Money(1.1);

because that's still creating a double literal before going to the Money object, which already implies a loss of precision.

A 'Gotcha' on Equals

Since Money is really just a wrapper for Java's BigDecimal, Money's equals method internally calls the equals method on BigDecimal. The gotcha here is that the equals method on BigDecimal compares both value and scale. This allows for the following interesting results:

assert(BigDecimal.ZERO.equals(new Money(new BigDecimal("0.0")).getAmount()) == false);

This especially comes into play when comparing values obtained from entities, since BigDecimal columns (like retailPrice and salePrice on Sku) are annotated like this:

    @Column(name = "RETAIL_PRICE", precision=19, scale=5)
    protected BigDecimal retailPrice;

    @Column(name = "SALE_PRICE", precision=19, scale=5)
    protected BigDecimal salePrice;

So if you were ever trying to see if the salePrice or retailPrice was zero, BigDecimal.ZERO.equals(sku.getSalePrice().getAmount()) will always be false since the database enforces a scale of 5, and the BigDecimal.ZERO constant has a scale of 0.

To get around this, you can use the compareTo method which ignores precision.

Sku sku = new SkuImpl();
sku.setRetailPrice(new Money(BigDecimal.ZERO);
sku = catalogService.saveSku(sku);

//assume that the changes have been completely persisted and the EntityManager flushed

Sku dbSku = catalogService.findSkuById(sku.getId());
assert(dbSku.getRetailPrice().toString().equals("0.00000"));
assert(BigDecimal.ZERO.equals(sku.getSalePrice().getAmount()) == false);
assert(BigDecimal.ZERO.compareTo(sku.getSalePrice().getAmount()) == 0);