Instant plus with negative values

Question

I have this code

Instant now = Instant.now();
if (amountDays >= 0) {
    now = now.plus(amountDays, ChronoUnit.DAYS);
} else {
    now = now.minus(Math.abs(amountDays), ChronoUnit.DAYS);
}

And I was thinking to simplify it like this

Instant now = Instant.now();
now = now.plus(amountDays, ChronoUnit.DAYS);

However, I am unsure if plus works correctly with negative values or if that messes up the result.

Can I use plus like that, with possibly negative values?

Answer 1

plus with negative values

The plus method supports adding negative times to go back in time, from its documentation:

amountToAdd - the amount of the unit to add to the result, may be negative

So all good, you can use it like that and it will work as expected.

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Implementation

Little trivia, the current implementation of minus even delegates to plus with -amountToSubtract as value:

return (amountToSubtract == Long.MIN_VALUE
    ? plus(Long.MAX_VALUE, unit).plus(1, unit)
    : plus(-amountToSubtract, unit));

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Notes

In general, if you just want to go back in time, prefer using minus for readability.

In your particular case I would stick with just plus though to not bloat the code and logic unecessarily. Instead, prefer adding a comment

// amountDays may be negative

or ensure that your javadoc is clear about that.

Minor improvement, you can simplify your code from two statements to just one:

Instant now = Instant.now().plus(amountDays, ChronoUnit.DAYS);

Answer 2

Just take a look how minus is implemented

@Override
public Instant minus(long amountToSubtract, TemporalUnit unit) {
    return (amountToSubtract == Long.MIN_VALUE ? plus(Long.MAX_VALUE, unit).plus(1, unit) : plus(-amountToSubtract, unit));
}

Of course for readability sake it makes sense to use both plus and minus in proper situations but checking whether amountDays is greater or lower from 0 seems to be some kind of internal Instant.plus logic and is deifinitely not helping with readability of your code