不太好的方法：用synchronized實現

當其中一個線程一直持有鎖時，會重復執行 if 判斷，做無用功！

/**
 * 兩個線程交替打印0-100的奇偶數，
 * 用synchronized關鍵字實現
 */
public class WaitNotifyPrintOddEvenSyn {

    // 新建2個線程
    // 1個只處理偶數，第二個只處理奇數（用位運算）
    // 用synchronized來通信

    private static int count;

    private static final Object lock = new Object();

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (count < 100) {
                    synchronized (lock) {
                        if ((count & 1) == 0) {
                            System.out.println(Thread.currentThread().getName() + ":" + count++);
                        }
                    }
                }
            }
        }, "偶數線程").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (count < 100) {
                    synchronized (lock) {
                        if ((count & 1) == 1) {
                            System.out.println(Thread.currentThread().getName() + ":" + count++);
                        }
                    }
                }
            }
        }, "奇數線程").start();
    }
}

更好的實現方法：用wait和notify實現

/**
 * 兩個線程交替打印0-100的奇偶數，
 * 用wait和notify實現
 */
public class WaitNotifyPrintOddEvenWait {

    // 1.拿到鎖，就打印
    // 2.打印完，喚醒其它線程，自己休眠

    private static int count = 0;

    private static final Object lock = new Object();

    static class TurningRunner implements Runnable {

        @Override
        public void run() {
            while (count <= 100) {
                synchronized (lock) {
                    System.out.println(Thread.currentThread().getName() + ":" + count++);
                    lock.notify();
                    if (count <= 100) {
                        try {
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        new Thread(new TurningRunner(), "偶數線程").start();
        new Thread(new TurningRunner(), "奇數線程").start();
    }
}