Threads - Beginning Java - page 785

A task that executes in a thread and returns a result must implement the Callable<> interface that declares
the call() method. The result that the call() method returns is the result of executing the task. You can
execute a Callable<> task in a thread pool like this:
Future<String> future = threadPool.submit(callableTask);
This statement executes the task, callableTask , and the future object encapsulates the result of execut-
ing the task that, in this instance, is an object of type String .
All versions of the submit() method throw an exception of type NullPointerException if the task ar-
gument is null and throw an exception of type java.util.concurrent.RejectedExecutionException
if the task cannot be executed for some reason.
Future<V> Object Methods
The Future<> reference that the submit() method returns is returned immediately, before the task com-
pletes execution. The Future<> object therefore provides a way to access the executing task and determine
its state. You can use the Future<> object to cancel execution of the task to which it relates:
if(future.cancel(true)) {
System.out.println("Task has been cancelled.");
} else {
System.out.println("Task could not be cancelled.");
}
The argument to the cancel() method should be specified as true if the task should be interrupted. If
you specify the argument as false and the task is still executing, it is allowed to continue. The method re-
turns false if the task could not be cancelled and true otherwise.
You can call the isCancelled() method for a Future<> object to determine whether or not the task was
canceled before it completed normally. The method returns true if the task was canceled and false other-
wise. If the task has ended, calling the isDone() method returns true ; it returns false if the task is still
running.
You obtain the result of executing a task by calling the get() method for the Future<> object that relates
to it. There are two versions of get() . The first has no parameters and you call it like this:
String result = null;
try {
result = future.get();
} catch(CancellationException e) {
System.out.println("Task was cancelled before a result was obtained.");
} catch(InterruptedException e) {
System.out.println(
"Current thread was interrupted while awaiting a result.");
} catch(ExecutionException e) {
System.out.println("Task threw an exception: ");
Throwable cause = e.getCause();
if(cause == null) {
System.out.println("Cause could not be determined.");
} else {
System.out.println(cause);
}