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Java Callable Example

By Arvind Rai, January 09, 2014
java.util.concurrent.Callable has been introduced in JDK 5 . Callable is a thread that returns the result. There is a method call() in Callable interface that must be overridden for computation task. To run Callable, submit() method of ExecutorService is used. ExecutorService also provides invokeAll() and invokeAny () method to run Callable threads. To fetch the result of call() method of Callable interface, java provides Future class. ExecutorService.submit() method returns Future instance and then get() method of Future, returns the result of call() method of Callable.

call() in Callable Interface

call() method returns the result. The result can be object of the class which has been declared in class declaration. The basic purpose of call() method is computation of result. This is same as run() method of Runnable interface but returns the result.

How to Run Callable in Java

To use Callable, we are creating a class which is implementing Callable interface. The class is overriding call() method.
package com.concretepage.util.concurrent;
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 CallableDemo {
   public static void main(String[] args) throws InterruptedException, ExecutionException {
	   ExecutorService service =  Executors.newSingleThreadExecutor();
	   SumTask sumTask = new SumTask(20);
	   Future<Integer> future = service.submit(sumTask);
	   Integer result = future.get();
class SumTask implements Callable<Integer> {
	private int num = 0;
        public SumTask(int num){
   	        this.num = num;
	public Integer call() throws Exception {
		int result = 0;
		for(int i=1;i<=num;i++){
		return result;
Inside call method, we are doing a simple task. A number is passed and for that number we are generation a summation of numbers from 1 to that given number. The submit() method of ExecutorService class is submitting the Callable object to run. The returned result from Callable is obtained as Future object. get() method of Future object returns the result of call() method of Callable object.

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