Finding the biggest number with ForkJoin

Question

Without block if delay == 2 it works, but doesn't work with it.

Here's ForkJoin's compute:

compute() {

    if(lastElem - firstElem == 1)
        return array[firstElem];

    if(lastElem - firstElem == 2){

        if(array[firstElem] > array[lastElem])
            return array[firstElem];
        else
            return array[lastElem];
    }

    int midElem = (firstElem + lastElem) / 2;

    MyForkJoinTask left = new MyForkJoinTask(array, firstElem, midElem);
    MyForkJoinTask right = new MyForkJoinTask(array, midElem, lastElem);

    left.fork();
    right.fork();

    int leftResult = left.join();
    int rightResult = right.join();

    if(leftResult > rightResult)
        return leftResult;
    else
        return rightResult;


}

Here's my main method:

main(String[] args) {
    Random r = new Random();
    for(int i = 0; i < array.length; i++){
        array[i] = r.nextInt();
    }
    MyForkJoinTask root = new MyForkJoinTask(array);
    F_J_POOL.submit(root);
    System.out.println("Result is " + root.join());
}

Short error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException

at com.bg.fork_join.MainFJ.main(MainFJ.java:24) Caused by: java.lang.ArrayIndexOutOfBoundsException ... 15 more

Caused by: java.lang.ArrayIndexOutOfBoundsException: 10000 at com.bg.fork_join.MyForkJoinTask.compute(MyForkJoinTask.java:31)

Could you show me where the problem is?

Answer 1

I don't know anything about ForkJoin, but this looks suspicious just on general principles:

if(lastElem - firstElem == 2){

    if(array[firstElem] > array[lastElem])
        return array[firstElem];
    else
        return array[lastElem];
}

The rest of your code makes it appear that you're processing a subarray from firstElem (inclusive) to lastElem (exclusive), so shouldn't you be using lastElem-1 to index?