java.util.concurrent.Callable对象可以返回由线程完成的计算结果，而runnable接口只能运行线程。 Callable对象返回Future对象，该对象提供监视线程执行的任务进度的方法。 Future对象可用于检查Callable的状态，然后线程完成后从Callable中检索结果。 它还提供超时功能。
语法
//submit the callable using ThreadExecutor
//and get the result as a Future object
Future result10 = executor.submit(new FactorialService(10));
//get the result using get method of the Future object
//get method waits till the thread execution and then return the result of the execution.
Long factorial10 = result10.get();
实例
以下TestThread程序显示了基于线程的环境中Futures和Callables的使用。
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class TestThread {
public static void main(final String[] arguments) throws InterruptedException, ExecutionException {
ExecutorService executor = Executors.newSingleThreadExecutor();
System.out.println("Factorial Service called for 10!");
Future<Long> result10 = executor.submit(new FactorialService(10));
System.out.println("Factorial Service called for 20!");
Future<Long> result20 = executor.submit(new FactorialService(20));
Long factorial10 = result10.get();
System.out.println("10! = " + factorial10);
Long factorial20 = result20.get();
System.out.println("20! = " + factorial20);
executor.shutdown();
}
static class FactorialService implements Callable<Long>{
private int number;
public FactorialService(int number) {
this.number = number;
}
@Override
public Long call() throws Exception {
return factorial();
}
private Long factorial() throws InterruptedException{
long result = 1;
while (number != 0) {
result = number * result;
number--;
Thread.sleep(100);
}
return result;
}
}
}
这将产生以下结果。
Factorial Service called for 10!
Factorial Service called for 20!
10! = 3628800
20! = 2432902008176640000
