Getting a result in the future?

You could write a method, that kicks of some long running task asynchronously. You would then return a future object, that is empty but gets filled when the long running task is completed. In other programming languages, this is called a promise.

Here is an simple example. I created a method called someLongAsyncOperation which executes something that takes a while. To simulate this, I just sleep for 3 seconds before generating an answer.

import java.util.UUID;
import java.util.concurrent.*;

public class Test {

    private static final ExecutorService executorService = Executors.newSingleThreadExecutor();

    public Future<MyAnswer> someLongAsyncOperation(){

        Future<MyAnswer> future = executorService.submit(() -> {
            Thread.sleep(3000);
            return new MyAnswer(UUID.randomUUID().toString());
        });

        return future;
    }


    public static void main(String[] args) throws Exception {

        System.out.println("calling someLongAsyncOperation ...");
        Future<MyAnswer> future = new Test().someLongAsyncOperation();
        System.out.println("calling someLongAsyncOperation done.");

        // do something else

        System.out.println("wait for answer ...");
        MyAnswer myAnswer = future.get();
        System.out.printf("wait for answer done. Answer is: %s", myAnswer.value);

        executorService.shutdown();
    }

    static class MyAnswer {
        final String value;

        MyAnswer(String value) {
            this.value = value;
        }
    }
  }

If you execute this little test class, you'll see, that someLongAsyncOperation returns fast, but when calling future.get(); we wait for the operation to complete.

You could now do something like starting of more than one longAsyncOperation, so they would run in parallel. And then wait until all of them are done.

Does this work as a starting point for you?

EDIT

You could implement someMethod like this:

public MyAnswer someMethod() throws ExecutionException, InterruptedException {
        Future<MyAnswer> future = someLongAsyncOperation(); // kick of async operation
        return future.get(); // wait for result
    }

Which will make the async operation synchron again, by calling it and waiting for the result.

EDIT2

Here's another example that uses wait/notify:

import java.util.UUID;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Test2 {

    private static final ExecutorService executorService = Executors.newSingleThreadExecutor();
    private Object receivedObject;
    private final Object mutex = new Object();

    public static void main (String[] args) throws InterruptedException {
        Object obj = new Test2().someMethod();

        System.out.println("The object is" + obj + ", wooh!");

        executorService.shutdown();
    }

    public void callObject() {

        System.out.println("callObject ...");

        // Sends request for the object asynchronously!
        executorService.submit(() -> {

            // some wait time to simulate slow request
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            // provide object to callback
            receiveObject(UUID.randomUUID().toString());
        });

        System.out.println("callObject done.");
    }

    public void receiveObject(Object object) {

        System.out.println("receiveObject ...");

        synchronized (mutex) {
            this.receivedObject = object;
            mutex.notify();
        }

        System.out.println("receiveObject done.");
    }

    public Object someMethod() throws InterruptedException {

        System.out.println("someMethod ...");

        synchronized (mutex) {
            callObject();
            while(this.receivedObject == null){
                mutex.wait();
            }
        }

        System.out.println("someMethod done.");
        return this.receivedObject;
    }

}

someMethod waits until receivedObject exists. receiveObject notifies upon arrival.