public interface Callable<V>
{
V call() throws Exception;
}
public interface Future<V>
{
V get() throws . . .;
V get(long timeout, TimeUnit unit) throws . . .;
void cancel(boolean mayInterrupt);
boolean isCancelled();
boolean isDone();
}
无参数的get方法会一直blocked直到计算完成，有参数如果时间到了还没完成，则抛出TimeoutException。他们两个都会抛出 InterruptedException
cancel方法试图中止任务线程。如果线程还没有start()那么它永不会开始，如果已经开始并且cancel的参数为true,则会调用interrupt()方法来中止
Callable<Integer> myComputation = . . .;
FutureTask<Integer> task = new FutureTask<Integer>(myComputation);
Thread t = new Thread(task); // it's a Runnable
t.start();
. . .
Integer result = task.get(); // it's a Future
