Returning value from Thread

I have a method with a HandlerThread. A value gets changed inside the Thread and I'd like to return it to the test() method. Is there a way to do this?

public void test()
{   
    Thread uiThread = new HandlerThread("UIHandler"){
        public synchronized void run(){
            int value; 
            value = 2; //To be returned to test()
        }
    };
    uiThread.start();
}

Usually you would do it something like this

 public class Foo implements Runnable {
     private volatile int value;

     @Override
     public void run() {
        value = 2;
     }

     public int getValue() {
         return value;
     }
 }

Then you can create the thread and retrieve the value (given that the value has been set)

Foo foo = new Foo();
Thread thread = new Thread(foo);
thread.start();
thread.join();
int value = foo.getValue();

tl;dr a thread cannot return a value (at least not without a callback mechanism). You should reference a thread like an ordinary class and ask for the value.

You can use a local final variable array. The variable needs to be of non-primitive type, so you can use an array. You also need to synchronize the two threads, for example using a CountDownLatch:

public void test()
{   
    final CountDownLatch latch = new CountDownLatch(1);
    final int[] value = new int[1];
    Thread uiThread = new HandlerThread("UIHandler"){
        @Override
        public void run(){
            value[0] = 2;
            latch.countDown(); // Release await() in the test thread.
        }
    };
    uiThread.start();
    latch.await(); // Wait for countDown() in the UI thread. Or could uiThread.join();
    // value[0] holds 2 at this point.
}

You can also use an Executor and a Callable like this:

public void test() throws InterruptedException, ExecutionException
{   
    ExecutorService executor = Executors.newSingleThreadExecutor();
    Callable<Integer> callable = new Callable<Integer>() {
        @Override
        public Integer call() {
            return 2;
        }
    };
    Future<Integer> future = executor.submit(callable);
    // future.get() returns 2 or raises an exception if the thread dies, so safer
    executor.shutdown();
}

What you are looking for is probably the Callable<V> interface in place of Runnable, and retrieving the value with a Future<V> object, which also lets you wait until the value has been computed. You can achieve this with an ExecutorService, which you can get from Executors.newSingleThreadExecutor() .

public void test() {
    int x;
    ExecutorService es = Executors.newSingleThreadExecutor();
    Future<Integer> result = es.submit(new Callable<Integer>() {
        public Integer call() throws Exception {
            // the other thread
            return 2;
        }
    });
    try {
        x = result.get();
    } catch (Exception e) {
        // failed
    }
    es.shutdown();
}

How about this solution?

It doesn't use the Thread class, but it IS concurrent, and in a way it does exactly what you request

ExecutorService pool = Executors.newFixedThreadPool(2); // creates a pool of threads for the Future to draw from

Future<Integer> value = pool.submit(new Callable<Integer>() {
    @Override
    public Integer call() {return 2;}
});

Now all you do is say value.get() whenever you need to grab your returned value, the thread is started the very second you give value a value so you don't ever have to say threadName.start() on it.

What a Future is, is a promise to the program, you promise the program that you'll get it the value it needs sometime in the near future

If you call .get() on it before it's done, the thread that's calling it will simply just wait until it's done

From Java 8 onwards we have CompletableFuture. On your case, you may use the method supplyAsync to get the result after execution.

Please find some reference here.

    CompletableFuture<Integer> completableFuture
      = CompletableFuture.supplyAsync(() -> yourMethod());

   completableFuture.get() //gives you the value