用Java编写求π包含并行和串行

下面是Java编写的串行和并行求π的示例代码：

串行求π：

public class SerialPi {
    public static void main(String[] args) {
        int n = 10000000;
        double pi = 0;
        int sign = 1;
        for (int i = 1; i <= n; i += 2) {
            pi += sign * 4.0 / i;
            sign = -sign;
        }
        System.out.println("π ≈ " + pi);
    }
}

并行求π：

public class ParallelPi {
    public static void main(String[] args) throws InterruptedException {
        int n = 10000000;
        int numThreads = 4;
        double pi = 0;
        Thread[] threads = new Thread[numThreads];
        for (int i = 0; i < numThreads; i++) {
            final int threadId = i;
            threads[i] = new Thread(() -> {
                double subSum = 0;
                int sign = (threadId % 2 == 0) ? 1 : -1;
                for (int j = threadId + 1; j <= n; j += numThreads) {
                    subSum += sign * 4.0 / j;
                    sign = -sign;
                }
                synchronized (ParallelPi.class) {
                    pi += subSum;
                }
            });
            threads[i].start();
        }
        for (int i = 0; i < numThreads; i++) {
            threads[i].join();
        }
        System.out.println("π ≈ " + pi);
    }
}

这里使用的方法和参数与之前提到的示例代码相同，区别在于并行计算使用了多个线程来计算级数的不同部分，并使用 synchronized 关键字来保证各个线程对共享变量 pi 的访问是安全的。