How will you increment each element of an Integer array, using parallel operation

Carvia Tech | April 23, 2019 | 2 min read | 347 views

Java 7 ForkJoinPool, Java 8 Arrays.parallelSetAll can be used for performing parallel operations on an integer array.

Using ForkJoinPool

We can use ForkJoin framework to divide this task recursively into multiple sub tasks and fork them using multiple cpu’s available to the JVM. Java 7 provides us with RecursiveAction class that can be extended to utilize ForkJoinPool framework with a great ease.

Dividing task recursively

IncrementTask.java

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;

class IncrementTask extends RecursiveAction {
    private final int THRESHOLD = 100;
    final long[] array;
    final int lo, hi;

    IncrementTask(long[] array, int lo, int hi) {
        this.array = array;
        this.lo = lo;
        this.hi = hi;
    }

    protected void compute() {
        if (hi - lo < THRESHOLD) {
            for (int i = lo; i < hi; ++i)
                array[i]++;
        } else {
            int mid = (lo + hi) >>> 1;
            invokeAll(new IncrementTask(array, lo, mid), new IncrementTask(array, mid, hi));
        }
    }

    public static void main(String[] args) {
        long[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        IncrementTask incrementTask = new IncrementTask(array, 0, 9);
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        forkJoinPool.invoke(incrementTask);
        System.out.println("array = " + Arrays.toString(array));
    }
}

Program output

array = [2, 3, 4, 5, 6, 7, 8, 9, 10]

Using Array parallel operation in Java 8

Java 8 provides great support for parallel operations on array using standard library functions.

Java 8 style array parelle operation

long[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9};
System.out.println("array = " + Arrays.toString(array));
Arrays.parallelSetAll(array, x -> array[x] + 1);    (1)
System.out.println("array = " + Arrays.toString(array));

1	`x` is the current index of array

Program output

array = [1, 2, 3, 4, 5, 6, 7, 8, 9]
array = [2, 3, 4, 5, 6, 7, 8, 9, 10]

As we can see Java 8 style parallel operations are much simpler than plain ForkJoinPool implementation.

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