Part 5: java.util.concurrent - Submit Callable< T > via ExecutorService

Future submit(Callable c)
Today in next part of the series we will talk about submission of callable task via executor service.

As per JAVA version 6.0, ExecutorService Interface has following method -
Future< T > submit( callable c )
Submits a Callable task for execution and returns a Future representing that task computation.
Future< T > will return T on successful completion of the process, otherwise result will not be T.
Lets start with Callable task - As we know that Callable interface has following method - public T call()
So when we say - ExecutorService.submit(Callable Task) --> It starts a new stack starting with call method of callable task, as call method can return T so does future object. Due to this reason the signature of submit(Callable c) is Future< T >.
T --> means anything that extends Object.
package com.jovialjava.blog.threads;

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 CallableExample {

private static final ExecutorService executorPool=Executors.newFixedThreadPool(2);

public static void main(String[] args) {
CallableTask_1 task_1 = new CallableTask_1();
CallableTask_2 task_2 = new CallableTask_2();

/**
* Submit the first task
*/
Future fut_1 = executorPool.submit(task_1);
Future fut_2 = executorPool.submit(task_2);

try{
if(fut_1.get()){
System.out.println("TASK 1 completed SUCCESSFULLY");
}
if(fut_2.get()){
System.out.println("TASK 2 completed SUCCESSFULLY");
}
}catch(ExecutionException e){
System.out.println(e.getMessage());
}catch(InterruptedException e){
System.out.println(e.getMessage());
}finally{
/**====VERY IMPORTANT===
* This is required to stop the executor pool to
* stop accepting new request.
*/
executorPool.shutdown();
}

}

/**
* This task will complete successfully
*/
public static class CallableTask_1 implements Callable{
public Boolean call()throws NullPointerException{
System.out.println("Hi, Inside Callable Task 1");
return true;
}

}

/**
* This task will result in error.
*/
public static class CallableTask_2 implements Callable{
public Boolean call()throws NullPointerException{
System.out.println("Hi, Inside Callable Task 2");
throw new IllegalStateException("Callable Task Exception");
}

}
}