think in java - concurrency - Callable

# When to use Callable?

If you want the task to produce a value when it's done.

public class CallableDemo {
	public static void main(String[] args) {
		ExecutorService pool = Executors.newCachedThreadPool();
		List<Future<String>> results = new ArrayList<Future<String>>();
		for (int i = 0; i < 10; i++) {
			results.add(pool.submit(new TaskWithResult(i)));
		}
		
		for (Future<String> future : results) {
			try {
				System.out.println(future.get());
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (ExecutionException e) {
				e.printStackTrace();
			} finally {
				pool.shutdown();
			}
		}
	}
}

class TaskWithResult implements Callable<String> {
	
	private int id;
	
	public TaskWithResult(int id) {
		this.id = id;
	}
	
	@Override
	public String call() throws Exception {
		return "id: " + id;
	}
}

# Future.isDone()

query the Future object to see if it has completed.

＃Future.get()

when the task is done, you can fetch the result using get(), or the call to get() will block until the result is ready.

# how to convert a Runnable into Callable?

Executors.callable(runnable);

Executors.callable(runnable, result);