java8parallelstream

• Z时代
• 2024-01-10
• 分类：综合

Parallel stream

Fork and join

Pseudocode

If(task is small enough or no longer divisible){

Compute task sequentially

}else{

split task in two subtasks

call this method recursively possibly further splitting  each subtask

wait for the completion of all subtasks

combine the results of each subtask}

public class ForkJoinSumCalculator extends RecursiveTask<Long> {

    private final long[] numbers;
    private final int start;
    private final int end;
    public static final long THRESHOLD = 10_1000;


    public ForkJoinSumCalculator(long[] numbers) {
        this(numbers, 0, numbers.length);

    }

    public ForkJoinSumCalculator(long[] numbers, int start, int end) {
        this.numbers = numbers;
        this.start = start;
        this.end = end;

    }


    @Override
    protected Long compute() {

        int length = end - start;
        if (length <= THRESHOLD) {
            return computeSequentially();
        }
        ForkJoinSumCalculator leftTask = new ForkJoinSumCalculator(numbers, start, start + length / 2);
        leftTask.fork();
        ForkJoinSumCalculator rightTask = new ForkJoinSumCalculator(numbers, start + length / 2, end);
        Long rightResult = rightTask.compute();
        Long leftResult = leftTask.join();
        return leftResult + rightResult;
    }

    private long computeSequentially() {
        long sum = 0;
        for (int i = start; i < end; i++) {
            sum += numbers[i];
        }
        return sum;
    }

    public static long forkJoinSum(long n){
        long[] numbers= LongStream.rangeClosed(1,n).toArray();
        ForkJoinTask<Long> task=new ForkJoinSumCalculator(numbers);
        return new ForkJoinPool().invoke(task);
    }
}

@Test
public void test(){
    long i = 10000000;
    StopWatch stopWatch=new StopWatch();
    stopWatch.start();
    Function<Long,Long> adder=ForkJoinSumCalculator::forkJoinSum ;
    adder.apply(i);
    stopWatch.stop();
    System.out.println(stopWatch.getTotalTimeSeconds());
}

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