Question

I am building a web app in Java that does math and shows steps to the user. When doing basic arithmetic with decimals I often get the messy in accurate outputs.

Here is my problem:

double a = 0.15;
double b = 0.01;
System.out.println(a - b);
// outputs 0.13999999999999999

float a = 0.15;
float b = 0.01;
System.out.println(a - b);
// outputs 0.14

float a = 0.16f;
float b = 0.01f;
System.out.println(a - b);
// outputs 0.14999999

double a = 0.16;
double b = 0.01;
System.out.println(a - b);
// outputs 0.15

Neither is reliable for complete accuracy. Is there a numeric class that is more precise or should I just round the values off?

Answer 1

You can use BigDecimal for this. It’s ugly, but it works:

BigDecimal a = new BigDecimal("0.15");
BigDecimal b = new BigDecimal("0.01");
System.out.println(a.subtract(b));

Be sure to construct them either with a String parameter, or with the valueOf method, like this:

BigDecimal x = new BigDecimal("0.15");   // This is ok
BigDecimal x = BigDecimal.valueOf(0.15); // This is also ok

And not with a double parameter, like this:

BigDecimal x = new BigDecimal(0.15); // DON'T DO THIS

Because if you pass in a double, you will also pass in double’s inaccuracy into the new BigDecimal instance. If you pass in a String, BigDecimal will know™ and do the right thing™.