Pass value from Threads in java

Question

I have one task which should be done by two thread at the same time.I am going to send one array to both threads to calculate something (For example thread 1 should check half of array and return result and thread two should check the other half and return result) and I want to add the result of both threads as final result in my main

class loop1 extends Thread {
    int sum1 = 0;
    ArrayList < Integer > list;
    public loop1(ArrayList < Integer > lis) {
        this.list = list;
    }
    public void run() {
        try {
            for (int i = 0; i < list.size() / 2; i++) {
                sum1 += lis.get(i);
            }
        } catch(Exception e) {}
    }
    return sum1;
}

class loop2 extends Thread {
    int sum2 = 0;
    ArrayList < Integer > list;
    public loop1(ArrayList < Integer > lis) {
        this.list = list;
    }
    public void run() {
        try {
            for (int i = lis.size() / 2; i < list.size(); i++) {
                sum1 += lis.get();
            }
        } catch(Exception e) {}
    }
    return sum2;
}

class check {
    public static void main(String[] args) {
        ArrayList < Integer > list = new ArrayList < >();
        loop1 loop1 = new loop1(lis);
        loop2 loop2 = new loop2(list);
        loop1.start();
        loop2.start();
        int sum = sum1 + sum2;
        System.out.print(sum);
    }
}

Answer 1

Main points

Threads are started not in the same way they were running.

loop1.start();
loop2.start();
// here you should check that both loop1 and loop2 thread has been finished
int sum = loop1.sum1 + loop2.sum2;

---

Using ExecutionService and Feature

public class Check {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService pool = Executors.newFixedThreadPool(2);
        int[] arr = createArray(100);

        Future<Integer> task1 = calculate(arr, 0, arr.length / 2, pool);
        Future<Integer> task2 = calculate(arr, arr.length / 2, arr.length, pool);

        while (!task1.isDone() || !task2.isDone()) {
            Thread.sleep(300);
        }

        System.out.println(task1.get() + task2.get());
    }

    private static int[] createArray(int length) {
        int[] arr = new int[length];

        for (int i = 0; i < arr.length; i++)
            arr[i] = i;

        return arr;
    }

    private static Future<Integer> calculate(final int[] arr, final int fromInclusive, final int toExclusive, ExecutorService pool) {
        return pool.submit(() -> {
            int sum = 0;

            for (int i = fromInclusive; i < toExclusive; i++)
                sum += arr[i];

            return sum;
        });
    }
}

---

Using class Thread

public class Check {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        int[] arr = createArray(100);
        Task task1 = new Task(arr, 0, arr.length / 2);
        Task task2 = new Task(arr, arr.length / 2, arr.length);

        task1.start();
        task2.start();

        task1.join();
        task2.join();

        System.out.println(task1.getSum() + task2.getSum());
    }

    private static int[] createArray(int length) {
        int[] arr = new int[length];

        for (int i = 0; i < arr.length; i++)
            arr[i] = i;

        return arr;
    }

    private static final class Task extends Thread {
        private final int[] arr;
        private final int fromInclusive;
        private final int toExclusive;
        private int sum;

        public Task(int[] arr, int fromInclusive, int toExclusive) {
            this.arr = arr;
            this.fromInclusive = fromInclusive;
            this.toExclusive = toExclusive;
        }

        public int getSum() {
            return sum;
        }

        @Override
        public void run() {
            for (int i = fromInclusive; i < toExclusive; i++)
                sum += arr[i];
        }
    }
}