I am wondering if there is a good way to return an object from a running thread. In my android project (not important for the question) I have this method:

public void getFolders()
{ Thread t = new Thread(new Runnable() { @Override public void run() { List<File> result = new ArrayList<File>(); Files.List request = null; do { try { request = service.files().list(); request.setQ("'appdata' in parents"); FileList files = request.execute(); result.addAll(files.getItems()); request.setPageToken(files.getNextPageToken()); } catch (IOException e) { System.out.println("An error occurred: " + e); request.setPageToken(null); } } while (request.getPageToken() != null && request.getPageToken().length() > 0); } }); t.start();
}

This method grabs some data from the internet and store the result in List<File> result. That's why I do not want to run it in the UI thread. Now I want to return this List to my main method. What is the best way to do this?

4

2 Answers

public interface Callable<V>

A task that returns a result and may throw an exception. Implementors define a single method with no arguments called call.

The Callable interface is similar to Runnable, in that both are designed for classes whose instances are potentially executed by another thread. A Runnable, however, does not return a result and cannot throw a checked exception.

How to use Callable.

EDIT:Also you should be using AsyncTask in android for doing background tasks and not create threads of your own.

1

You should use Callable interface instead of Runnable interface to create threads. Callable interface offers a call() method, which can return an Object.

Because you cannot pass a Callable into a Thread to execute, you instead use the ExecutorService to execute the Callable object. The service accepts Callable objects to run by way of the submit() method:

 <T> Future<T> submit(Callable<T> task)

As the method definition shows, submitting a Callable object to the ExecutorService returns a Future object. The get() method of Future will then block until the task is completed.

You can follow the sample on this link and customize it according to your requirement: