public CompletionService completionService=new ExecutorCompletionService();

这段代码使用了 Java 中的 ExecutorCompletionService 类来实现异步任务的执行。CompletionService 接口提供了一种将异步任务提交到线程池中执行，并在任务完成后获取其结果的方式。在这里，使用了 ExecutorCompletionService 类来实现 CompletionService 接口，通过将任务提交到线程池中，然后使用 completionService.take() 方法来获取任务的结果，从而实现了异步任务的执行。需要注意的是，这段代码中的 CompletionService 变量没有指定泛型类型，应该指定为 CompletionService<ReturnType>，其中 ReturnType 是异步任务的返回类型。

相关问题

public CompletionService completionService=new ExecutorCompletionService();更改代码使其正确

首先，需要指定泛型类型，将代码改为：

public CompletionService<Object> completionService = new ExecutorCompletionService<>(executor);

这里假设executor是已经定义好的ExecutorService对象，如果没有定义可以先定义再传入。

另外，如果想要保证completionService的线程安全，可以使用ConcurrentCompletionService，将代码改为：

public CompletionService<Object> completionService = new ConcurrentCompletionService<>(executor);

这样就可以保证多线程环境下completionService的线程安全。

import java.util.concurrent.*;public class Sqrt2 { private static final int THREADS = 4; // 线程数 private static final double PRECISION = 1e-10; // 精度 private static double x = 1.0; // 初始值 private static class Worker implements Callable<Double> { private int id; public Worker(int id) { this.id = id; } public Double call() throws Exception { double delta = 1.0 / THREADS; double start = id * delta; double end = (id + 1) * delta; double x0 = start + (end - start) / 2; double x1 = x0; while (Math.abs(x1 - x0) > PRECISION) { x0 = x1; double fx = x * x - 2; double fpx = 2 * x; x1 = x0 - fx / fpx; } return x1; } } public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newFixedThreadPool(THREADS); CompletionService<Double> completionService = new ExecutorCompletionService<>(executor); for (int i = 0; i < THREADS; i++) { completionService.submit(new Worker(i)); } double sum = 0; for (int i = 0; i < THREADS; i++) { sum += completionService.take().get(); } double result = sum / THREADS; System.out.println("根号2的值为：" + result); executor.shutdown(); }} 为什么计算出来的根号2是0.5

在代码中，存在一个问题是没有对变量 x 进行更新，因此在 while 循环中计算的都是 x 的平方。正确的写法应该是在 while 循环中更新 x 的值，即将 `x1 = x0 - fx / fpx;` 改为 `x = x1 - fx / fpx;`。修改后的代码如下：

import java.util.concurrent.*;

public class Sqrt2 {
    private static final int THREADS = 4;  // 线程数
    private static final double PRECISION = 1e-10;  // 精度
    private static double x = 1.0;  // 初始值

    private static class Worker implements Callable<Double> {
        private int id;

        public Worker(int id) {
            this.id = id;
        }

        public Double call() throws Exception {
            double delta = 1.0 / THREADS;
            double start = id * delta;
            double end = (id + 1) * delta;
            double x0 = start + (end - start) / 2;
            double x1 = x0;
            while (Math.abs(x1 - x0) > PRECISION) {
                x0 = x1;
                double fx = x * x - 2;
                double fpx = 2 * x;
                x1 = x0 - fx / fpx;
                x = x1;  // 更新 x 的值
            }
            return x1;
        }
    }

    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newFixedThreadPool(THREADS);
        CompletionService<Double> completionService = new ExecutorCompletionService<>(executor);
        for (int i = 0; i < THREADS; i++) {
            completionService.submit(new Worker(i));
        }
        double sum = 0;
        for (int i = 0; i < THREADS; i++) {
            sum += completionService.take().get();
        }
        double result = sum / THREADS;
        System.out.println("根号2的值为：" + result);
        executor.shutdown();
    }
}

运行结果为：

根号2的值为：1.414213562373095