How to update an Atomic based on a condition?

Question

How to update an AtomicInteger if its current value is less than the given value? The idea is:

AtomicInteger ai = new AtomicInteger(0);
...
ai.update(threadInt); // this call happens concurrently
...
// inside AtomicInteger atomic operation
synchronized {
    if (ai.currentvalue < threadInt)
        ai.currentvalue = threadInt;
}

Your posted code snippet make no sense. You first update `ai` (whatever you mean with .update()) and after you check the value with the new one. — SubOptimal, Apr 14 '15 at 11:55
This is a good question and it makes perfect sense. For the criticism: It is impossible for the OP to write working code for this. If he did, he would need to know the answer, so he can only post pseudo-code. — Daniel S., Apr 14 '15 at 12:00

score 23 · Accepted Answer · answered Apr 14 '15 at 11:58

If you are using Java 8 you can use one of the new update methods in AtomicInteger, which you can pass a lambda expression. For example:

AtomicInteger ai = new AtomicInteger(0);

int threadInt = ...

// Update ai atomically, but only if the current value is less than threadInt
ai.updateAndGet(value -> value < threadInt ? threadInt : value);

score 5 · Answer 2 · answered Mar 22 '18 at 09:56

If I didn't have Java 8, I would probably create a utility method, something like:

public static boolean setIfIncreases(AtomicInteger ai, int newValue) {
    int currentValue;
    do {
        currentValue = ai.get();
        if (currentValue >= newValue) {
            return false;
        } 
     } while (!ai.compareAndSet(currentValue, newValue));
     return true;
}

From the OP's code, it would then be invoked thus:

AtomicInteger ai = new AtomicInteger(0);

int threadInt = ...

// Update ai atomically, but only if the current value is less than threadInt
setIfIncreases(ai, threadInt);

score 3 · Answer 3 · answered Apr 14 '15 at 12:02

3

If you don't have Java 8, you can use a CAS-loop like this :

while (true) {
    int currentValue = ai.get();
    if (newValue > currentValue) {
        if (ai.compareAndSet(currentValue, newValue)) {
            break;
        }
    }
}

answered Apr 14 '15 at 12:02

Olivier Croisier

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I clicked -1, because this code causes an infinite loop if newValue <= currentValue. – Mikko Östlund Mar 22 '18 at 08:52

How to update an Atomic based on a condition?

3 Answers3