Runnable和Thread都没有返回值，如果想要在线程结束后有返回值，需要Callable接口。Callable是个支持泛型的接口，我们是实现这个接口的时候可以指定返回值得类型。
其实我们还需要一个支持取得返回值得方式，并且实现了Runnable的类。这个类就是FutureTask。FutureTask就是一个Runnable，因此可以调用Thread的start()方法运行它。
例子：

package thread;

import java.text.SimpleDateFormat;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

class MyCallable implements Callable<String>{

    @Override
    public String call() throws Exception {
        Thread.sleep(1000*10);
        return "OK";
    }
    
}
public class TestCallable {

    public static void main(String[] args) {
        FutureTask<String> featureTask =new FutureTask<String>(new MyCallable());
        new Thread(featureTask).start();
        System.out.println(formatDate(System.currentTimeMillis()));
        try {
            System.out.println("The result is "+ featureTask.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        System.out.println(formatDate(System.currentTimeMillis()));
    }
    
    private static String formatDate(long date){
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        return simpleDateFormat.format(date);
    }

}

执行结果：

2017/06/22 22:39:12
The result is OK
2017/06/22 22:39:22

从结果看到，get()方法是个同步方法，会阻塞程序执行。

另一种有返回值的方式就是 executorService.submit()，submit一个实现Callable接口的类就行，这种方式get也是会阻塞的。

package thread;

import java.text.SimpleDateFormat;
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;

class MyCallable implements Callable<String> {

    @Override
    public String call() throws Exception {
        Thread.sleep(1000 * 10);
        return "OK";
    }

}

public class TestCallable {

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        Future<String> future = executorService.submit(new MyCallable());

        System.out.println(formatDate(System.currentTimeMillis()));

        try {
            System.out.println("The result is " + future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        System.out.println(formatDate(System.currentTimeMillis()));
    }

    private static String formatDate(long date) {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        return simpleDateFormat.format(date);
    }

}

结果相似，只是时间不同