Using float and double for calculation, giving different results [duplicate]

Question

I am working on a project where we are calculating prices of items with 8 places of decimal points. But sometime calculation results are different.

I tried some samples but getting different calculation results:

    public static void main(String[] args) {
        double value1 =  0.999939f * 0.987792f;
        double value2 =  0.999939 * 0.987792;
        double value3 =  (double)0.999939f * 0.987792f;     
        System.out.println("value 1 : "+ value1);
        System.out.println("value 2: "+ value2);
        System.out.println("value 3 : "+ value3);
    }

Outputs are:

value 1 : 0.9877317547798157
value 2 : 0.9877317446880001
value 3 : 0.9877317839126931

These three are different results.

I am confused. Can anyone please clarify me what is happening here?
Thanks.

I went through some already answered, But I just want some ways where I can calculate with float and doubles only. otherwise I have to change many places. It will be painful.

Answer 1

Because of how floats and decimals are represented, the casts could be causing different results.

For: double value1 = 0.999939f * 0.987792f you are multiplying two floats and then casting it to a double. The resulting float representation is the one being converted to a double.

For: double value2 = 0.999939 * 0.987792; you are multiply two doubles and saving it in a double. This is time there is no casting so the representation never changes (ie. no potential data loss from change in data representation).

For: double value3 = (double)0.999939f * 0.987792f; you are multiplying a double that is casted from a float, times a float. Because of how the Java math works, that second float is probably also being casted to a double, so now you are multiplying two doubles that were once floats causing a third set of representations.

Because float and doubles have different precisions each of these will get a different result. For more info about floating point arithmetic, see here.

Answer 2

When you write a number with the "f" character, it is taken as float, meaning it is encoded with 32bits, whereas without it, it is a double, encoded with 64bits.

The more bits, the more accurately you will represent decimal numbers. This does not make a difference for number with few decimals, but in your case it is significant.

In conclusion, use exclusively double variables in your code.