Java Multithreading - how to use wait() and notify() to get a value from only 1 thread out of 2 and kill the other thread

Question

In my main thread I create two additional threads that I want to use a value from. Basically what I want to do is this:

Threads thread1 = new Threads();
Threads thread2 = new Threads();
Thread.currentThread.wait();
If (thread1 = complete){
var = thread1.getter
//delete thread2
}
If (thread2 == complete){
var = thread2.getter
//delete thread1
}

With thread1 and thread2 having a notify() at the end that wakes up the main thread and the thread that doesn't finish is deleted. But I realise that I don't properly understand wait() and multithreading so the way this is setup may not be correct. I know that Thread.currentThread.wait() is definitely not correct.

I think I may have to synchronize the methods but I have not been able to find any examples that show how to do this in this situation.

Edit: To give more info Thread1 takes input from a scanner and Thread2 takes input from a keylistener and I want to use the first input from 1 of them

wait and notify and fairly low level operations that act as the basis for some other concurrency primitives. IMO you as an application developer should almost never be using them directly. — Michael, Dec 01 '22 at 16:06
Re, `Thread.currentThread.wait()`, The [javadoc for Thread.join()](https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/lang/Thread.html#join()) says, "It is recommended that applications _not_ use `wait`, `notify`, or `notifyAll` on `Thread` instances." — Solomon Slow, Dec 01 '22 at 17:19

score 0 · Answer 1 · answered Dec 01 '22 at 16:20

0

You don't need wait and notify in your case. Just use join to wait until the thread is completed.

More info: Java Wait for thread to finish

answered Dec 01 '22 at 16:20

Vadim Beskrovnov

941
6
18

I think OP wants to wait for whichever thread finishes _first,_ and then tell the other threads to give up. There is no `join()` method that awaits more than one thread. – Solomon Slow Dec 01 '22 at 17:22

Solomon Slow · Accepted Answer · 2022-12-01T20:51:18.863

I think you are saying that your threads are racing to solve a problem, and you are only interested in whichever solution is produced first. Am I right? If so, then what you need is some shared variables that allow all the threads to know if a solution has been found:

boolean solution_is_found = false;
SolutionType solution;
final Object solution_mutex = new Object();

A worker thread can do this:

while (true) {
    synchronized(solution_mutex) {
        if (solution_is_found) {
            // Some other thread won the race, our solution is not needed.
            return;
        }
    }
    ...do some increment of work...
    if (...this thread found a solution...) {
        synchronized(solution_mutex) {
            if (! solution_was_found) {
                // Only update the variables if this thread won the race.
                solution_is_found = true;
                solution = ...what this thread found...
                solution_mutex.notifyAll();
            }
        }
        return;
    }
}

And now, the main thread can await the appearance of the first solution.

synchronized(solution_lock) {
    while (! solution_was_found) {
        solution_lock.wait();
    }
}

...use `solution` here...

If the smallest possible "increment of work" takes a significant chunk of time, then you may want to test solution_is_found several times within the "increment," and abort if it is true.

Exactly right, I only care about whichever value was calculated first. Thank you very much for this. — StupidQuestion, Dec 01 '22 at 21:45

Java Multithreading - how to use wait() and notify() to get a value from only 1 thread out of 2 and kill the other thread

2 Answers2