Recently I am studying the concurrent concept on Java. And I wrote a demo code to help understand how `ExecutorService` works. First, the code below is the worker code. It implements the `Callable` interface, and define a task to do. The simple task `MyTask` create is a task to print out a sentence on the console and return the `taskNum` assigned on initialization. ```java import java.util.Random; import java.util.concurrent.Callable; public class MyTask implements Callable<Integer>{ private int taskNum; public MyTask(int taskNum){ this.taskNum = taskNum; } @Override public Integer call() throws Exception { // TODO Auto-generated method stub Random random = new Random(); System.out.println("This is `call()` method of Task #" + taskNum); Thread.sleep(random.nextInt(500)); return taskNum; } } ``` We assign 30 tasks with different `taskNum` to the `ExecutorService` object and store the `Future` object, that is the result of our task, in an `ArrayList`. And finally, we try to get the result of all the tasks one by one. ```java import java.util.ArrayList; import java.util.List; import java.util.Random; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; public class ExecutorServiceDemo { public static void main(String[] args) throws InterruptedException { Random random = new Random(); List<Future<Integer>> results = new ArrayList<Future<Integer>>(); ExecutorService executor = Executors.newFixedThreadPool(2); for (int i = 0; i < 30; i ++){ MyTask thisTask = new MyTask(i); results.add(executor.submit(thisTask)); } executor.shutdown(); System.out.println("Successfully distribute all the tasks!"); System.out.println("Waiting for the results!"); for (Future<Integer> result: results){ try { System.out.println("Result: " + result.get(random.nextInt(500) , TimeUnit.MILLISECONDS)); } catch (Exception e) { System.out.println("Result Timeout, drop result of this task"); } } } } ``` Here is one of the possible result: ```text This is `call()` method of Task #0 This is `call()` method of Task #1 Successfully distribute all the tasks! Waiting for the results! Result: 0 This is `call()` method of Task #2 Result Timeout, drop result of this task Result: 2 This is `call()` method of Task #3 This is `call()` method of Task #4 Result: 3 This is `call()` method of Task #5 Result Timeout, drop result of this task This is `call()` method of Task #6 Result Timeout, drop result of this task This is `call()` method of Task #7 Result: 6 This is `call()` method of Task #8 Result: 7 This is `call()` method of Task #9 Result Timeout, drop result of this task This is `call()` method of Task #10 Result Timeout, drop result of this task This is `call()` method of Task #11 Result: 10 This is `call()` method of Task #12 Result Timeout, drop result of this task Result: 12 This is `call()` method of Task #13 This is `call()` method of Task #14 Result: 13 This is `call()` method of Task #15 This is `call()` method of Task #16 Result: 14 Result: 15 This is `call()` method of Task #17 Result Timeout, drop result of this task Result Timeout, drop result of this task This is `call()` method of Task #18 Result: 18 This is `call()` method of Task #19 This is `call()` method of Task #20 This is `call()` method of Task #21 Result: 19 This is `call()` method of Task #22 Result: 20 Result Timeout, drop result of this task Result: 22 This is `call()` method of Task #23 Result Timeout, drop result of this task This is `call()` method of Task #24 This is `call()` method of Task #25 Result: 24 This is `call()` method of Task #26 This is `call()` method of Task #27 Result: 25 Result: 26 This is `call()` method of Task #28 Result: 27 This is `call()` method of Task #29 Result: 28 Result Timeout, drop result of this task ```