多线程一定要比单线程快吗

public class ThreadTest {
    
    private static final long count = 100000;

    private static void concurrency() throws InterruptedException {
        long startTime = System.currentTimeMillis();
        Thread thread = new Thread(() -> {
            long a = 0;
            for (long i = 0; i < count; i++) {
                a += 2;
            }
        });
        thread.start();
        int b = 0;
        for (long i = 0; i < count; i++) {
            b++;
        }
        thread.join();
        long time = System.currentTimeMillis() - startTime;
        System.out.println("concurrency thread spend time: " + time);
    }

    private static void singleThread() {
        long startTime = System.currentTimeMillis();
        long a = 0;
        for (long i = 0; i < count; i++) {
            a += 2;
        }
        int b = 0;
        for (long i = 0; i < count; i++) {
            b++;
        }
        long time = System.currentTimeMillis() - startTime;
        System.out.println("single thread spend time: " + time);
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadTest.concurrency();
        ThreadTest.singleThread();
    }
}

结论：多线程不一定要比单线程要快
原因分析：线程有创建和上下文切换需要花费时间

1.无锁并发编程：多线程竞争锁时，会引起上下文切换，使用一些方法来避免加锁。
2.CAS算法：使用Java中Atomic包
3.使用最少线程：避免大量创建线程，任务很少就不要创建大量线程，会造成大量线程处于等待状态
4.协程：在单线程里实现多任务的调度，并在单线程多个任务间的切换

部分资料参考《Java并发编程的艺术》方腾飞 魏鹏 程晓明