Modulus with doubles in Java

Question

How do you deal with Java's weird behaviour with the modulus operator when using doubles?

For example, you would expect the result of 3.9 - (3.9 % 0.1) to be 3.9 (and indeed, Google says I'm not going crazy), but when I run it in Java I get 3.8000000000000003.

I understand this is a result of how Java stores and processes doubles, but is there a way to work around it?

Answer 1

Use a precise type if you need a precise result:

    double val = 3.9 - (3.9 % 0.1);
    System.out.println(val); // 3.8000000000000003

    BigDecimal x = new BigDecimal( "3.9" );
    BigDecimal bdVal = x.subtract( x.remainder( new BigDecimal( "0.1" ) ) );
    System.out.println(bdVal); // 3.9

Why 3.8000...003? Because Java uses the FPU to calculate the result. 3.9 is impossible to store exactly in IEEE double precision notation, so it stores 3.89999... instead. And 3.8999%0.01 gives 0.09999... hence the result is a little bit bigger than 3.8.