The accuracy of a double in general programming and Java

Question

I understand that due to the nature of a float/double one should not use them for precision important calculations. However, i\'m a little confused on their limitations due

Answer 1

I suggest you read this page:
https://en.wikipedia.org/wiki/Double-precision_floating-point_format

Once you've read and understood it, and perhaps converted several examples to their binary representations in the 64 bit floating point format, then you'll have a much better idea of what significant digits a Double can hold.

Answer 2

As a side note, (perhaps trivial) a nice and reliable way to store a known precision of value is to simply multiply it by the relevant factor and store as some integral type, which are completely precise.

For example:

double costInPounds = <something>; //e.g. 3.587
int costInPence = (int)(costInPounds * 100 + 0.5); //359

Plainly some precision can be lost, but if a required/desired precision is known, this can save a lot of bother with floating point values, and once this has been done, no precision can be lost by further manipulations.

The + 0.5 is to ensure that rounding works as expected. (int) takes the 'floor' of the provided double value, so adding 0.5 makes it round up and down as expected.