Java — Callable and Future | Code Factory

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• In the case of Runnable job Thread wouldn’t return anything after completing a job.
• If a thread is require to return some result after execution then we should go for Callable.
• Callable interface contains only 1 method call().

Object call() throws Exception

• If we submit Callable object to Executor then after completing the job thread returns an Object of the type Future.
• That is Future object can be used to retrieve the result from Callable job.

package com.example.thread;
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;
/**
 * @author code.factory
 *
 */
public class CallableTest {
	public static void main(String... args) throws InterruptedException, ExecutionException {
		MyCallable jobs[] = {
			new MyCallable(10),
			new MyCallable(20),
			new MyCallable(30),
			new MyCallable(40),
			new MyCallable(50),
			new MyCallable(60),
		};
		ExecutorService service = Executors.newFixedThreadPool(3);
		for(MyCallable job : jobs) {
			Future f = service.submit(job);
			System.out.println(f.get());
		}
		service.shutdown();
	}
}
class MyCallable implements Callable {
	int no;
	public MyCallable(int no) {
		this.no = no;
	}
	
	public Object call() throws Exception {
		System.out.print(Thread.currentThread().getName() + " sum of first " + no + " numbers : ");
		int sum = 0;
		for(int i=0; i<no; i++) {
			sum = sum + i;
		}
		return sum;
	}
}

Output :

pool-1-thread-1 sum of first 10 numbers : 45
pool-1-thread-2 sum of first 20 numbers : 190
pool-1-thread-3 sum of first 30 numbers : 435
pool-1-thread-1 sum of first 40 numbers : 780
pool-1-thread-2 sum of first 50 numbers : 1225
pool-1-thread-3 sum of first 60 numbers : 1770