用Fork and Join实现归并排序

很早就知道JDK7的ForkAndJoin框架了，一直没有学习。看了官网WordCount的例子，
http://www.oracle.com/technetwork/articles/java/fork-join-422606.html，
自己写一个归并排序练练手。(代码中增加的打印语句完全是为了方便理解)

import java.util.*;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class MergeSortTask extends RecursiveTask<int[]> {
    private static final String ROOT = "0";
    private static final String LEFT = "0";
    private static final String RIGHT = "1";
    private final int[] array;
    private final String serial;

    public MergeSortTask(int[] array, String serial) {
        this.array = array;
        this.serial = serial;
    }

    @Override
    protected int[] compute() {
        int length = array.length;
        if (length <= 3) {
            Arrays.sort(array);
            return array;
        }

        String leftSerial = getLeftSerial();
        String rightSerial = getRightSerial();

        int[] leftArray = Arrays.copyOfRange(array, 0, length / 2);
        int[] rightArray = Arrays.copyOfRange(array, length / 2, length);
        MergeSortTask leftTask = new MergeSortTask(leftArray, leftSerial);
        MergeSortTask rightTask = new MergeSortTask(rightArray, rightSerial);

        System.out.println(new MidArray(leftSerial, leftArray));
        System.out.println(new MidArray(rightSerial, rightArray));

        int[] leftResult = leftTask.fork().join();
        int[] rightResult = rightTask.fork().join();

        System.out.println(new MidResult(new MidArray(leftSerial, leftResult)));
        System.out.println(new MidResult(new MidArray(rightSerial, rightResult)));

        return merge(leftResult, rightResult);
    }

    private String getLeftSerial() {
        return serial + "-" + LEFT;
    }

    private String getRightSerial() {
        return serial + "-" + RIGHT;
    }

    private int[] merge(int[] leftArray, int[] rightArray) {
        int leftArrayLength = leftArray.length;
        int rightArrayLength = rightArray.length;
        int[] resultArray = new int[leftArrayLength + rightArrayLength];

        int resultIndex = 0;
        int leftIndex = 0;
        int rightIndex = 0;

        while (leftIndex < leftArrayLength && rightIndex < rightArrayLength) {
            if (leftArray[leftIndex] <= rightArray[rightIndex]) {
                resultArray[resultIndex++] = leftArray[leftIndex++];
            } else {
                resultArray[resultIndex++] = rightArray[rightIndex++];
            }
        }

        //copy left rest
        if (leftIndex < leftArrayLength) {
            System.arraycopy(leftArray, leftIndex, resultArray, 
                                 resultIndex, leftArrayLength - leftIndex);
        }
        //copy right rest
        if (rightIndex < rightArrayLength) {
            System.arraycopy(rightArray, rightIndex, resultArray, 
                                resultIndex, rightArrayLength - rightIndex);
        }

        return resultArray;
    }

    private static int[] buildTestArray(int size) {
        int[] testArray = new int[size];
        Random rand = new Random();
        for (int i = 0; i < size; i++) {
            testArray[i] = rand.nextInt(100);
        }

        return testArray;
    }

    public static void main(String[] args) {
        int[] testArray = buildTestArray(10);
        System.out.println(Arrays.toString(testArray));
        MergeSortTask mergeSorter = new MergeSortTask(testArray, ROOT);
        ForkJoinPool pool = new ForkJoinPool();
        System.out.println(Arrays.toString(pool.invoke(mergeSorter)));
    }
}

//辅助类
import java.util.Arrays;

public class MidArray {
    private final String serial;
    private final int[] result;
    private static final String INDENT = "  ";

    public MidArray(String serial, int[] result) {
        this.serial = serial;
        this.result = result;
    }

    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < serial.split("-").length; i++) {
            sb.append(INDENT);
        }
        sb.append(serial).append(", is: ").append(Arrays.toString(result));

        return sb.toString();
    }
}

public class MidResult {

    private MidArray midArray;

    public MidResult(MidArray midArray) {
        this.midArray = midArray;
    }

    public String toString() {
        return midArray.toString() + " [Result]";
    }
}

一次运行的输出结果：

[71, 8, 25, 9, 4, 41, 38, 87, 45, 96]
    0-0, is: [71, 8, 25, 9, 4]
    0-1, is: [41, 38, 87, 45, 96]
      0-0-0, is: [71, 8]
      0-0-1, is: [25, 9, 4]
      0-0-0, is: [8, 71] [Result]
      0-0-1, is: [4, 9, 25] [Result]
      0-1-0, is: [41, 38]
      0-1-1, is: [87, 45, 96]
      0-1-0, is: [38, 41] [Result]
      0-1-1, is: [45, 87, 96] [Result]
    0-0, is: [4, 8, 9, 25, 71] [Result]
    0-1, is: [38, 41, 45, 87, 96] [Result]
[4, 8, 9, 25, 38, 41, 45, 71, 87, 96]