Java Callable interface and Future interface

Introduction

The concurrent API Callable interface represents a thread that returns a value.

We can use Callable objects to do calculation then return to the invoking thread.

Future is a generic interface representing the value be returned by a Callable object.

The following program illustrates Callable and Future by creating three tasks that perform three different computations. 

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 Main {
  public static void main(String args[]) {
    ExecutorService es = Executors.newFixedThreadPool(3);
    Future<Integer> f;
    Future<Double> f2;
    Future<Integer> f3;

    System.out.println("Starting");

    f = es.submit(new Sum(10));
    f2 = es.submit(new Hypot(3, 4));
    f3 = es.submit(new Factorial(5));

    try {//  w  w  w . j a  v  a 2 s  .c  o m
      System.out.println(f.get());
      System.out.println(f2.get());
      System.out.println(f3.get());
    } catch (InterruptedException exc) {
      System.out.println(exc);
    } catch (ExecutionException exc) {
      System.out.println(exc);
    }

    es.shutdown();
    System.out.println("Done");
  }
}

// Following are three computational threads.

class Sum implements Callable<Integer> {
  int stop;

  Sum(int v) {
    stop = v;
  }

  public Integer call() {
    int sum = 0;
    for (int i = 1; i <= stop; i++) {
      sum += i;
    }
    return sum;
  }
}

class Hypot implements Callable<Double> {
  double side1, side2;

  Hypot(double s1, double s2) {
    side1 = s1;
    side2 = s2;
  }

  public Double call() {
    return Math.sqrt((side1 * side1) + (side2 * side2));
  }
}

class Factorial implements Callable<Integer> {
  int stop;

  Factorial(int v) {
    stop = v;
  }

  public Integer call() {
    int fact = 1;
    for (int i = 2; i <= stop; i++) {
      fact *= i;
    }
    return fact;
  }
}



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