ForkJoin算法实现

要求

一个数组内有10万个30左右的数值（非零），要求计算这些值的乘积。
- 时间要求：2s
- 堆内存大小：4m

实现方案

通过ForkJoin实现。

代码实现

import com.google.common.base.Joiner;
import com.google.common.base.Splitter;
import org.apache.commons.lang3.RandomUtils;

import java.math.BigInteger;
import java.util.Arrays;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;

/**
 * 一个数组内有10万个30左右的数值（非零），要求计算这些值的乘积。
 * 时间要求：2s
 * 堆内存大小：4m
 * <p>
 * 实现方案：通过ForkJoin实现。
 */
public class ForkJoinMultiply extends RecursiveTask<BigInteger> {
    private int[] numbers;
    private static final int THREASHOLD = 10000;
    private static final int MAX_SIZE = 100000;

    public ForkJoinMultiply(int[] numbers) {
        this.numbers = numbers;
    }

    @Override
    protected BigInteger compute() {
        if(numbers.length<=THREASHOLD){//如果数组容量小于阈值，直接计算乘积。
            BigInteger result = BigInteger.ONE;
            for(int i = 0;i<numbers.length;i++){
                result = result.multiply(new BigInteger(String.valueOf(numbers[i])));
            }
            return result;
        }else {//如果容量大于阈值，则分组计算。分成两组
            int middle = numbers.length/2;
            int[] left = Arrays.copyOfRange(numbers,0,middle);
            int[] right = Arrays.copyOfRange(numbers,middle,numbers.length);
            ForkJoinMultiply leftTask = new ForkJoinMultiply(left);
            ForkJoinMultiply rightTask = new ForkJoinMultiply(right);
            leftTask.fork();
            rightTask.fork();
            return leftTask.join().multiply(rightTask.join());
        }
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        BigInteger temp = BigInteger.ONE;
        int[] numbers = new int[MAX_SIZE];
        for(int i = 0;i<MAX_SIZE;i++){
            int randomNumber = RandomUtils.nextInt(20,40);
            temp = temp.multiply(BigInteger.valueOf(randomNumber));
            numbers[i] = randomNumber;
        }
        System.out.println("multiply result is "+temp+", cost time "+(System.currentTimeMillis()-start));

        start = System.currentTimeMillis();
        // 执行ForkJoin需要用到ForkJoinPool，调用commonPool方法是JDK1.8的实现
        ForkJoinPool forkJoinPool = ForkJoinPool.commonPool();
        ForkJoinMultiply multiply = new ForkJoinMultiply(numbers);
        BigInteger result = forkJoinPool.invoke(multiply);
        System.out.println("forkjoin multiply result is "+result+", cost time "+(System.currentTimeMillis()-start));
    }
}