Merge sort function using fork/join pool.

DivideTask.java

package nz.co.aetheric.tools.async;

import java.util.List;

import static com.google.common.collect.Lists.transform;
import static com.google.common.collect.Lists.partition;
import static com.google.common.collect.Lists.newArrayList;
import static com.google.common.collect.Iterables.mergeSorted;

public class DivideTask<E> extends RecursiveTask<List<E>> {
	
	final List<E> input;
	final Comparator<E> comparator;
	
	public DivideTask(List<E> input, Comparator<E> comparator) {
		this.input = input;
		this.comparator = comparator;
	}
	
	@Override
	protected E compute() {
		
		if (input.size() < 2) {
			return input.iterator().next();
		}

		final List<DivideTask<List<E>>> forks = new List<>();
		for (List<E> list : partition(input, input.size())) {
			forks.add(new DivideTask(list).fork(), comparator);
		}
		
		return newArrayList(mergeSorted(transform(forks, (DivideTask<List<E>> fork) -> fork.join()), comparator));

	}
	
}

MergeSort.java

package nz.co.aetheric.tools.async;

public final class MergeSort {
	
	public static <E> List<E> sort(List<E> list, Comparator<E> comparator) {
		return new ForkJoinPool().invoke(new DivideTask(list, comparator));
	}
	
	private MergeSort() {}
	
}