多线程一定快吗

下面的代码演示串行和并发执行并累加操作的时间，可以发现：当并发累加次数不超过百万次的时候，速度会比串行执行累加操作的要慢，因为线程有创建和上下文切换的开销。

package com.bingfa;public class ConcurrencyTest {    private static final long COUNT = 100001;    public static void main(String[] args) throws InterruptedException {        Concurrency();        serial();    }    /**     * currentTimeMillis() --> Returns the current time in milliseconds.     */    private static void serial() {        long start = System.currentTimeMillis();        int a = 0;        for (long i = 0; i < COUNT; i++) {            a += 5;        }        int b = 0;        for (long i = 0; i < COUNT; i++) {            b--;        }        long time = System.currentTimeMillis() - start;        System.out.print("serial: " + time + "ms, b=" + b + ",a= " + a);    }    /**     * InterruptedException:     * Thrown when a thread is waiting, sleeping, or otherwise occupied,     * and the thread is interrupted, either before or during the activity.     * Occasionally a method may wish to test whether the current thread has been interrupted,     * and if so, to immediately throw this exception.     *     * @throws InterruptedException      * 这个异常一般发生在线程中，当一个正在执行的线程被中断时就会出现这个异常     * 假如有两个线程，第一个线程正在运行，第二个没有运行，这时第二个线程启动运行并要求中断第一个线程，     * 第一个线程就会出现InterruptedException异常并执行该异常下的语句。     */    private static void Concurrency() throws InterruptedException {        long start = System.currentTimeMillis();        /* Thread类的构造方法中，需要一个实现了Runnable接口的对象，而new就是生成了个实现Runnable接口的类的一个实例对象。把这个实例作为Thread的参数         * 接口不能实例化，new Runnable()是一个实现接口Runnable的类的对象，后面的run方法是该类里实现的方法，这是匿名内部类的写法         */        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                int a = 0;                for (long i = 0; i < COUNT; i++) {                    a += 5;                }            }        });        /* 线程必须要先start，才能join，只有启动了，才能对线程进行操作 */        thread.start(); // 启动thread线程        int b = 0;        for (int i = 0; i < COUNT; i++) {            b--;        }        long time = System.currentTimeMillis() - start;        /* join的话则是将该线程加入到调用线程（一般为主线程） 等该线程结束后 调用线程才会继续运行 */        thread.join(); // 邀请thread线程先执行，本线程先暂停执行，等待thread线程执行完后，主线程再接着往下执行        System.out.println("concurrency : " + time + "ms,b=" + b);    }}

结果如下

concurrency : 3ms,b=-100001serial: 2ms, b=-100001,a= 500005Process finished with exit code 0

如果把COUNT=1000001
结果：

concurrency : 6ms,b=-1000001serial: 7ms, b=-1000001,a= 5000005Process finished with exit code 0

如果COUNT=100000001

concurrency : 233ms,b=-100000001serial: 595ms, b=-100000001,a= 500000005Process finished with exit code 0