Java asynchronously call a method for target output

Question

Suppose I have a blocking method called check as follows:

boolean check(String input) {}

which will do some check against the input and return the decision.

Now I want to run this check against a list of inputs asynchronously, and I want to return to the main thread right after one of the inputs passing the check, so I don't have to wait for all asynchronous call to be completed. The only one scenario for waiting all threads to complete is that there are no inputs passing the check. Running the method asynchronously with list of inputs is straightforward but I'm not sure how to return to the main thread after getting the target output for the input passing the check.

Answer 1

Here is a really simple working example to achieve what you are asking for

Future<Boolean> future = CompletableFuture.runAsync(() -> {
    // Do your checks, if true, just return this future
    System.out.println("I'll run in a separate thread than the main thread.");
});

// Now, you may want to block the main thread waiting for the result
while(!future.isDone()) {
    // Meanwhile, you can do others stuff
    System.out.println("Doing other stuff or simply waiting...");
}

// When future.isDone() returns, you will be able to retrieve the result
Boolean result = future.get();

Answer 2

A basic parallelStream will do exactly that:

boolean match = inputs.parallelStream().anyMatch(input -> check(input));

Returns early with match==true, iff some input is found that matches check. match will be false if all inputs are checked and none matched.

In the standard case it will use the fork/join thread pool. But with some further effort you can avoid that.