June 16, 2011

Why your Java math might be wrong

We all know that mathematical operations with decimals in JavaScript are no good. For instance:

alert(1.1 + 2.2); // should be 3.3, but the result is 3.3000000000000003

It turns out this is a cross to bare for pretty much any language that uses the IEEE-754 standard to implement a floating point number and that includes... *drums*... Java.

// Adding doubles
System.out.println(1.1 + 2.2); // 3.3000000000000003

// Adding floats
System.out.println(1.1f + 2.2f); // 3.3000002

So if you're doing these kinds of operations with float or double in Java you might be doing them wrong.

This is huge because an application that deals with money is almost certain to have to deal with decimals. You have to get it right or you'll be in a lot of trouble.

"People have a reasonable expectation that when you’re adding up their money you’re going to come up with the right result, and you don’t" - Douglas Crockford

float and double should never be used for currency or high precision operations.

The proper way to do this is using the BigDecimal class. Check out the documentation.

Let's try it again, this time using BigDecimal:

BigDecimal num1 = new BigDecimal(1.1);
BigDecimal num2 = new BigDecimal(2.2);
System.out.println(num1.add(num2)); // 3.30000000000000026645352591003756...etc.

Oh noes! what happened? I thought BigDecimal was supposed to solve all my problems.

Hold on, take another look at the code above. You'll notice we're constructing our BigDecimal objects using a... *drums*... double.

So? Well, that's the deal with float and double, it's why they should not be used in these operations. They cannot accurately represent numbers with decimals.

OK, how do we fix this? There's a constructor that takes a String argument (didn't you look at the documentation?). That's how we should always do it. Pass a String so you won't lose precision:

BigDecimal num3 = new BigDecimal("1.1");
BigDecimal num4 = new BigDecimal("2.2");
System.out.println(num3.add(num4)); // 3.3, finally

Perfect! now lawsuits won't be falling from the sky.

Thanks for reading and let me know your comments.

Representing money
Beware of floating point numbers