在Java中什么是可调用的

Question

标题几乎概括了它。

我想知道callable的概念和思想。我在这里读到一个关于可调用和可运行之间差异的问题。但没有人展示代码并详细说明callable是什么。我不想知道它们之间的区别。我想知道，

1. 何时使用，如何使用。

2. 当他们开始行动Android .

Answer 1

你可以检查这个例子:

在这个例子中，Callable task在一秒钟后返回执行该任务的线程的名称。我们使用Executor框架并行执行100个任务，并使用Future来获取提交任务的结果。

package com.journaldev.threads;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
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 MyCallable implements Callable<String> {
    @Override
    public String call() throws Exception {
        Thread.sleep(1000);
        //return the thread name executing this callable task
        return Thread.currentThread().getName();
    }
    public static void main(String args[]){
        //Get ExecutorService from Executors utility class, thread pool size is 10
        ExecutorService executor = Executors.newFixedThreadPool(10);
        //create a list to hold the Future object associated with Callable
        List<Future<String>> list = new ArrayList<Future<String>>();
        //Create MyCallable instance
        Callable<String> callable = new MyCallable();
        for(int i=0; i< 100; i++){
            //submit Callable tasks to be executed by thread pool
            Future<String> future = executor.submit(callable);
            //add Future to the list, we can get return value using Future
            list.add(future);
        }
        for(Future<String> fut : list){
            try {
                //print the return value of Future, notice the output delay in console
                // because Future.get() waits for task to get completed
                System.out.println(new Date()+ "::"+fut.get());
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
        //shut down the executor service now
        executor.shutdown();
    }
}

你也可以检查使用Callable从Runnables返回结果

Answer 2

Callable与Runnable类似，但它返回结果并可能抛出异常。当你期望异步任务返回结果时使用它们。

异步计算的返回结果用Future表示。

你可以用FutureTask(它实现了RunnableFuture和Future)来检查这个简单的例子

public static void main(String[] args) {
    // providing an anonymous callable to FutureTask
    RunnableFuture<String> future = new FutureTask<String>(
            new Callable<String>() {
                @Override
                public String call() throws InterruptedException {
                    System.out.println("sleeping");
                    Thread.sleep(2000);
                    System.out.println("returning");
                    return "hello-world";
                }
            });
    Thread t = new Thread(future);
    t.start();
    try {
        // the get Waits if necessary for the computation to complete
        System.out.println(future.get());
    } catch (InterruptedException | ExecutionException e) {
        e.printStackTrace();
    }
}