03. Callable and Future.md

Callable

• The Callable interface represent a thread that return a value.
• interface Callable<V>, V represents the type of data returned by the task.
• It defines only one method call().
• A Callable task is executed by an ExecutorService, by calling its submit method.
• This submit method is of the form <T> Future<T> submit(Callable<T> task).
• It returns result through an object of type Future.

Future

• Future is a generic interface that represents the value that will be returned by a Callable object.
• interface Future<V>, V specifies the type of the result.
• To obtain the value, we need to call Future's get() method.

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.function.BiFunction;
import java.util.function.Function;

class Utility{
    static Function<Integer,Integer> sum = (Integer stop) ->{
        int total = 0;
        for(int i=0;i<=stop;i++){
            total += i;
        }
        return total;
    };

    static Function<Integer,Integer> factorial = (Integer stop) ->{
        int fact = 1;
        for(int i=2;i<=stop;i++){
            fact *= i;
        }
        return fact;
    };

    static BiFunction<Double,Double,Double> getHypotenuse = (Double s1, Double s2)-> Math.sqrt(s1*s1 + s2*s2);
}

public class Main {

    public static void main(String[] args) {
        ExecutorService ex = Executors.newFixedThreadPool(3);


        Future<Integer> f1 = ex.submit(()-> Utility.sum.apply(10));
        Future<Double> f2 = ex.submit(()-> Utility.getHypotenuse.apply(3.0,4.0));
        Future<Integer> f3 = ex.submit(()-> Utility.factorial.apply(5));

        try{
            System.out.println(f1.get());
            System.out.println(f2.get());
            System.out.println(f3.get());
        } catch (ExecutionException | InterruptedException  e) {
            throw new RuntimeException(e);
        }
        ex.shutdown();
        System.out.println("DONE");
    }
}