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AY19/20 S2 Q5: Asynchronous Programming #333

Open jcng2308 opened 3 years ago

jcng2308 commented 3 years ago

Summary

Hello, is there anyone who can explain how to derive all these expected outputs?

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ShyamalSankar commented 3 years ago

Hello, if I am not mistaken, the join at printAsync(1) means that it will definitely be executed before the main thread ends. Hence, any output that does not contain 1 is not possible. I think 1 must also be the first one out.

Next, I think that the allOf would mean that before 4 is outputted, 2 and 3 must definitely be outputted first (in any order). So anything that has 4 without both 2 and 3 inside is incorrect. The main thread may complete with either 2 or 3 completing without both completing so it is possible for combinations like 13, 12, etc (and ofc just 1 itself).

Again this is just my thought process and may not be a 100 percent correct!

(BTW can I ask where did you find this paper? I couldnt find it on the database)

jcng2308 commented 3 years ago

Hello, if I am not mistaken, the join at printAsync(1) means that it will definitely be executed before the main thread ends. Hence, any output that does not contain 1 is not possible. I think 1 must also be the first one out.

Next, I think that the allOf would mean that before 4 is outputted, 2 and 3 must definitely be outputted first (in any order). So anything that has 4 without both 2 and 3 inside is incorrect. The main thread may complete with either 2 or 3 completing without both completing so it is possible for combinations like 13, 12, etc (and ofc just 1 itself).

Again this is just my thought process and may not be a 100 percent correct!

(BTW can I ask where did you find this paper? I couldnt find it on the database)

I just checked it is still in the database, i can send you the paper if you want! Thanks for the response anyway

ShyamalSankar commented 3 years ago

Its ok! I managed to get it. thanks!

jeremycte commented 3 years ago

For future reference to anyone visiting this thread. The answer are a, e & n are yes. The rest of the other outputs is no.

Some argue why wouldn't (o) be true. Reason being is coz printAsync(1).join(). Join statements must be printed before 2,3 or 4 can be completed.

----Edit---- As what Buwoo mentioned, 2 can output without 3 becoz of the method "allOf", refer to this link https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html Therefore, either 2 or 3 can go first and both can be completed or only one actually completing (vise versa).

KaveenE commented 3 years ago

@jeremycte If am not wrong, shouldn't e be no since we should have a '3' if we have '2'?

Buwoo commented 3 years ago

@jeremycte If am not wrong, shouldn't e be no since we should have a '3' if we have '2'?

2 can output without 3 actually completing, and vice versa. We only need both 2 and 3 to output if 4 outputs too, due to the allOf

Herrekt commented 3 years ago

The latter code requires 1 to be printed first before anything. For 4 to be printed, both 2 and 3 will need to be printed before it (in any order). 2 or 3 can be printed without affecting the other (after 1).

KaveenE commented 3 years ago

Got it, thanks!