How would I have a thread wait on a lambda? | Java Community | Help. Code. Learn. | Page 1

earnest crane Oct 31, 2024, 3:15 PM

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This is my first time messing with threads. The project takes user input with a lambda, so I need to wait until that lambda gets valid data to resume the thread. From my research I havent found a good way to do that other than while (var == null) {} which doesn't seem ideal.

blissful moatBOT Oct 31, 2024, 3:15 PM

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serene wing Oct 31, 2024, 6:27 PM

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what input are you taking?

earnest crane Oct 31, 2024, 6:28 PM

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Right now just a string from terminal, but thats for testing.

serene wing Oct 31, 2024, 6:29 PM

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well what do you mean by valid data?

earnest crane Oct 31, 2024, 6:29 PM

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oh it needs to be one of three strings(selecting an option)

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specifically im looking for one of "draft", "set", or "random"

serene wing Oct 31, 2024, 6:30 PM

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why don't you just check if the input is equal to one of those things ?

earnest crane Oct 31, 2024, 6:31 PM

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because I can't stop the main thread until the user gives input(This is eventally going to be server side code on something with multiple users)

serene wing Oct 31, 2024, 6:32 PM

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so let me see if I got this right

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you have a main thread which shouldn't stop running so you have delegated the user input to another thread and you are now wondering how to notify the main thread when valid input has been provided?

earnest crane Oct 31, 2024, 6:34 PM

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no, user input is handled on the main thread. I'm passing the thing that handles user input a lambda to run the message through and I want to continue the side thread when that lambda has gotten valid input

serene wing Oct 31, 2024, 6:35 PM

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you just said you can't stop the main thread until the user gives input

earnest crane Oct 31, 2024, 6:35 PM

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I'm not

serene wing Oct 31, 2024, 6:35 PM

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can the main thread block or can it not block?

earnest crane Oct 31, 2024, 6:35 PM

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the main thread is getting a lambda that extends a specific interface for it to run messages through

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as a simplified example: ```java
SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
switch (message.toLowerCase()) {
case "draft" -> type = SelectionType.DRAFT;
case "set" -> type = SelectionType.SET;
case "random" -> type = SelectionType.RANDOM;
default -> return false;
}
return true;
}
//use type for something

serene wing Oct 31, 2024, 6:39 PM

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who runs this function?

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is it the main thread or some other thread

earnest crane Oct 31, 2024, 6:39 PM

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its run on the main thread when it gets a message

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and it gets the message from side threads that are actually doing the listening for user input

serene wing Oct 31, 2024, 6:40 PM

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right so what is the problem with validating the input before sending the message off to the main thread?

earnest crane Oct 31, 2024, 6:42 PM

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the problem is I need to know that type is not null before the code continues, and that requires the specific user to send a valid message.

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the main thread can't just wait forever because then we wouldnt handle messages from other users

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so I need the code to wait until the value is set, and I'm not sure how to do that

serene wing Oct 31, 2024, 6:44 PM

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this is confusing

earnest crane Oct 31, 2024, 6:44 PM

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earnest crane so I need the code to wait until the value is set, and I'm not sure how to do th...

(well other than while (type == null) {})

serene wing Oct 31, 2024, 6:44 PM

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in the first place you can't set type from inside the lambda

earnest crane Oct 31, 2024, 6:44 PM

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serene wing in the first place you can't set type from inside the lambda

yeah I know, that was just quicker for my example

serene wing Oct 31, 2024, 6:45 PM

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how does the main thread handle this message?

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Is it running an event loop?

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like run me through what happens when some thread receives some data from somewhere

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why can't it validate that data before sending it off to the main thread

earnest crane Oct 31, 2024, 7:30 PM

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serene wing why can't it validate that data before sending it off to the main thread

So let me phrase this differently(I don’t know exactly how the main code works, I’m writing an addon with an api), all messages go through the main thread and this is triggered by a message. So for other messages to be processed the main thread has to be freed, and this needs another message before it can finish its task.

serene wing Oct 31, 2024, 7:42 PM

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so im assuming the main thread is ran by this api then?

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again, what is the problem with validating the input in the current thread before captureMessageFrom

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wait a second

serene wing Oct 31, 2024, 7:44 PM

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earnest crane the main thread is getting a lambda that extends a specific interface for it to ...

do you want the side thread to wait for type to be set in this example?

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in other words:

SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
    switch (message.toLowerCase()) {
        case "draft" -> type = SelectionType.DRAFT;
        case "set" -> type = SelectionType.SET;
        case "random" -> type = SelectionType.RANDOM;
        default -> return false;
    }
    return true;
}
//wait until type !=null
//use type for something

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is this what you are trying to do?

earnest crane Oct 31, 2024, 8:04 PM

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serene wing is this what you are trying to do?

Yes

earnest crane Oct 31, 2024, 8:04 PM

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serene wing again, what is the problem with validating the input in the current thread befor...

That lambda is where we get the input (it’s the message parameter), and it’s run on the main thread later

serene wing Oct 31, 2024, 8:05 PM

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okay

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you can do this multiple ways

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the simplest is with a CountDownLatch

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        SelectionType type;
        final CountDownLatch latch = new CountDownLatch(1);

        runner.captureMessagesFrom(user, (message) -> {
            try{
                switch (message.toLowerCase()) {
                    case "draft" -> type = SelectionType.DRAFT;
                    case "set" -> type = SelectionType.SET;
                    case "random" -> type = SelectionType.RANDOM;
                    default -> return false;
                }
                return true;                
            }finally {
                latch.countDown();
            }

        });
        try {
            latch.await();
        } catch (InterruptedException e) {
            //handle interrupt as you see fit
        }

//use type for something

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the thread will block on await() until another calls countDown()

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you can find out more about the CountDownLatch here:
https://docs.oracle.com/javase/6/docs/api/java/util/concurrent/CountDownLatch.html

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I suggest before jumping into multi-threaded code to have a look at synchronization blocks, locks and atomic variables

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it's also important to keep in mind that depending on your code the thread on await() might block indefinitely if the main thread shuts down before processing the message

serene wing Oct 31, 2024, 8:12 PM

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serene wing I suggest before jumping into multi-threaded code to have a look at synchronizat...

just learn a bit more about this before doing multi-threaded stuff

earnest crane Oct 31, 2024, 11:18 PM

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Thank you, this worked!

blissful moatBOT Oct 31, 2024, 11:18 PM

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earnest crane Thank you, this worked!

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#How would I have a thread wait on a lambda?