Question

I need to increment a float value atomically. I get its int value by calling Float.floatToIntBits on it. If I just do an i++ and convert it back to float, it does not give me the expected value. So how would I go about it?

(I’m trying to create an AtomicFloat through AtomicInteger, hence this question).

EDIT: here’s what I did:

Float f = 1.25f;
int i = Float.floatToIntBits(f);
i++;
f = Float.intBitsToFloat(i);

I wanted 2.25, but got 1.2500001 instead.

Answer 1

The reason is that the bits you get from floatToIntBits represents

1. sign
2. exponent
3. mantissa

laid out like this:

Repr:  Sign   Exponent          Mantissa
Bit:    31   30......23  22.....................0

Incrementing the integer storing these fields with 1 won’t increment the float value it represents by 1.

I’m trying to create an AtomicFloat through AtomicInteger, hence this question

I did precisely this in an answer to this question:

• Java: is there no AtomicFloat or AtomicDouble?

To add functionality to increment the float by one, you could copy the code of incrementAndGet from AtomicInteger (and change from int to float):

public final float incrementAndGet() {
    for (;;) {
        float current = get();
        float next = current + 1;
        if (compareAndSet(current, next))
            return next;
    }
}

(Note that if you want to increment the float by the smallest possible value, you take the above code and change current + 1 to current +Math.ulp(current).)