Callable Interface

Callable interface has one method call in it, it is just like run method of Runnable interface. But the main difference is, run method won't return any value where as call method returns a value.

In Some situation, it is necessary to return a value from a task and throw some exception in abnormal situations. But the run method in Runnable interface won't return any value and can't throw any exception. Java solve that problem by providing call method in Callable interface, call method returns any object and can throw an exception.

V call() throws Exception

Computes a result, or throws an exception if unable to do so. Returns the computed result.

CallableTask.java

import java.util.concurrent.*;
import java.util.*;

public class CallableTask implements Callable{
 String name;

 CallableTask(String name){
  this.name = name;
 }

 public String call(){
  try{
   Thread.sleep(1000);
  }
  catch(Exception e){
  }
  return (new Date() + " :: " + name);
 }
}

SimpleThreadPool.java

import java.util.concurrent.*;
import java.util.*;

class SimpleThreadPool{
 public static void main(String args[])throws Exception{
  List<Future<String>> l1 = new ArrayList<Future<String>> ();
  ExecutorService exec = new ThreadPoolExecutor(3, 3, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());

  CallableTask tasks[] =new CallableTask[10];

  for(int i=0; i<5; i++){
   tasks[i] = new CallableTask("task" + i);
   Future<String> future = exec.submit(tasks[i]);
   System.out.println("Submit the task " + i);
   l1.add(future);
  }

  exec.shutdown();
  exec.awaitTermination(10, TimeUnit.SECONDS);

  for(Future f1 : l1){
   System.out.println(f1.get());
  }
  System.out.println("Execution Completed");
 }
}

Output

Submit the task 0
Submit the task 1
Submit the task 2
Submit the task 3
Submit the task 4
Sat Mar 08 10:40:17 IST 2014 :: task0
Sat Mar 08 10:40:17 IST 2014 :: task1
Sat Mar 08 10:40:17 IST 2014 :: task2
Sat Mar 08 10:40:18 IST 2014 :: task3
Sat Mar 08 10:40:18 IST 2014 :: task4
Execution Completed

 ExecutorService exec = new ThreadPoolExecutor(3, 3, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())

Statement creates a thread pool of 3 threads.

exec.submit(tasks[i])

Submit the tasks to the thread pool. When the task is submitted, ThreadPool assigns a thread to execute the call method of the Callable Object.

exec.awaitTermination(10, TimeUnit.SECONDS)

Block the execution of the current thread for atmost 10 seconds, untill all the tasks finsishes. If all the tasks finishes before 10 seconds, it unblocks and proceed next statements executions.  

  

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