(MultiThreading/Parallel processing)

import java.util.concurrent.*;

public class Midterm_3 {

private Integer sum = new Integer(0);

public static void main(String[] args) {

Midterm_3 test = new Midterm_3();

System.out.println("What is sum ? " + test.sum);

}

public Midterm_3() {

ExecutorService executor = Executors.newFixedThreadPool(100);

Task task = new Task();

for (int i = 0; i < 15; i++) {

executor.execute(task);

}

executor.shutdown();

while(!executor.isTerminated()) {

}

}

class Task implements Runnable {

public void run() {

sum++;

}//end run

}//end Task

}//end Midterm_3

Running this code does not always give the expected result of 15. Why not, and what change can be made to the code so that it will perform as expected?