Java - Result-Targeting Tasks

Introduction

Callable<V> interface represents a task that can return a result upon its execution.

The type parameter V is type of the result of the task.

call() method from Callable interface can return a value of any type and can throw an exception.

It is declared as follows:

public interface Callable<V> {
        V call() throws Exception;
}

The following code shows how to use the Callable interface to create a task.

It uses a Callable Task with an Executor.

call() method contains the logic for task.

ExecutorService.submit() method returns a Future object.

Future interface tracks the progress of the task that you submit.

Demo

import java.util.Random;
import java.util.concurrent.Callable;

import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;

class MyTask implements Callable<Integer> {
  private int taskId;
  private int loopCounter;
  public MyTask(int taskId, int loopCounter) {
    this.taskId = taskId;
    this.loopCounter = loopCounter;
  }//from   w  w w . ja  v a2  s.c  o m

  public Integer call() throws InterruptedException {
    int totalSleepTime = 0;
    for (int i = 1; i <= loopCounter; i++) {
      try {
        int sleepTime = 3;
        System.out.println("Task #" + this.taskId + " - Iteration #" + i
            + " is going to sleep for " + sleepTime + " seconds.");
        Thread.sleep(sleepTime * 1000);
        totalSleepTime = totalSleepTime + sleepTime;
      } catch (Exception e) {
        System.out.println("Task #" + this.taskId + " has been interupted.");
        throw e;
      }
    }
    return totalSleepTime;
  }
}

public class Main {
  public static void main(String[] args) {
    ExecutorService exec = Executors.newFixedThreadPool(3);
    MyTask task = new MyTask(1, 3);
    Future<Integer> submittedTask = exec.submit(task);
    try {
      Integer result = submittedTask.get();
      System.out.println("Task's total sleep time: " + result + " seconds");
    } catch (ExecutionException e) {
      System.out.println("Error in executing the task.");
    } catch (InterruptedException e) {
      System.out.println("Task execution has been interrupted.");
    }
    exec.shutdown();
  }
}

Result

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